Here is an alphabetical list of the command line arguments processed by Gcal:
%date: Actual date modifier-# line: Fixed date options-*dn: Fixed date options-*dnwww: Fixed date options-*wn[www]: Fixed date options--adjust-value=argument: Fixed date options--alternative-format: Fixed date options--astronomical-holidays: Calendar options--atmosphere=air-pressure[,temperature]: Fixed date options--bahai-holidays: Calendar options--bahai-months: Calendar options--biorhythm-axis=number: Fixed date options--blocks=1: Calendar options--blocks=12: Calendar options--blocks=2: Calendar options--blocks=3: Calendar options--blocks=4: Calendar options--blocks=6: Calendar options--blocks=number: Calendar options--calendar-dates=both: Calendar options--calendar-dates=special: Calendar options--calendar-dates=special|both: Calendar options--cc-holidays=cc[_tt][+...]: Calendar options--celtic-holidays: Calendar options--chinese-flexible-holidays: Calendar options--chinese-flexible-months: Calendar options--chinese-holidays: Calendar options--chinese-months: Calendar options--christian-holidays: Calendar options--coptic-months: Calendar options--copyleft: Common options--copyright: Common options--cycle-end=argument: Fixed date options--cycle-step=argument: Fixed date options--date-format=de|us|gb|text: Calendar options--date-variable=argument: Fixed date options--debug=abort: Global options--debug=all: Global options--debug=handled: Global options--debug=internal: Global options--debug=unhandled: Global options--debug[=internal|handled|unhandled|all|abort]: Global options--descending-fixed-dates=long: Fixed date options--descending-fixed-dates=short: Fixed date options--descending-fixed-dates[=short|long]: Fixed date options--descending-holiday-list=long: Calendar options--descending-holiday-list=short: Calendar options--descending-holiday-list[=long|short]: Calendar options--disable-highlighting: Global options--end-of-month: Fixed date options--end-of-week: Fixed date options--end-of-year: Fixed date options--ethiopic-months: Calendar options--exclude-fixed-dates-list-title: Fixed date options--exclude-holiday-list-title: Calendar options--execute-command: Fixed date options--exit-status-help-non-zero: Common options--export-date-variables: Date variables--export-date-variables: Fixed date options--export-text-variables: Text variables--export-text-variables: Fixed date options--filter-day=argument: Fixed date options--filter-period=argument: Fixed date options--filter-text=pattern: Fixed date options--fixed-dates=both: Calendar options--fixed-dates=special: Calendar options--fixed-dates=special|both: Calendar options--force-highlighting: Global options--french-revolutionary-months: Calendar options--gregorian-reform=1582|1700|1752|1753|argument: Calendar options--grouping-text[=text]: Fixed date options--heading-text=text: Fixed date options--hebrew-holidays: Calendar options--hebrew-months: Calendar options--help: Invoking gcal2txt--help: Invoking txt2gcal--help: Invoking tcal--help: Common options--here=line: Fixed date options--highlighting=no: Global options--highlighting=text: Global options--highlighting=yes: Global options--holiday-dates=both: Calendar options--holiday-dates=special: Calendar options--holiday-dates=special|both: Calendar options--holiday-list=long: Calendar options--holiday-list=short: Calendar options--holiday-list[=long|short]: Calendar options--ignore-case: Fixed date options--include-consecutive-number: Fixed date options--include-holidays=long: Fixed date options--include-holidays=short: Fixed date options--include-resource-file-name: Fixed date options--include-today: Fixed date options--include-week-number: Fixed date options--indian-civil-months: Calendar options--islamic-civil-holidays: Calendar options--islamic-civil-months: Calendar options--iso-week-number=no: Calendar options--iso-week-number=yes: Calendar options--iso-week-number=yes|no: Calendar options--japanese-flexible-holidays: Calendar options--japanese-flexible-months: Calendar options--japanese-holidays: Calendar options--japanese-months: Calendar options--leap-day=february: Fixed date options--leap-day=february|march: Fixed date options--leap-day=march: Fixed date options--license: Common options--limit: Fixed date options--list-mode: Fixed date options--list-of-fixed-dates=long: Fixed date options--list-of-fixed-dates=short: Fixed date options--list-of-fixed-dates[=short|long]: Fixed date options--long-help=?: Common options--long-help[=argument]: Common options--mail[=address]: Global options--month: Fixed date options--moonimage-lines=number: Fixed date options--multicultural-new-year-holidays: Calendar options--old-armenic-months: Calendar options--old-egyptic-months: Calendar options--omit-multiple-date-part: Fixed date options--orthodox-calendar: Calendar options--orthodox-new-holidays: Calendar options--orthodox-old-holidays: Calendar options--pager: Global options--period-of-fixed-dates=argument: Fixed date options--persian-jalaali-holidays: Calendar options--persian-jalaali-months: Calendar options--precise: Fixed date options--resource-file=name[+...]: Fixed date options--response-file=name: Global options--revert-match: Fixed date options--shell-script=name: Global options--shift=[+|-]number: Invoking tcal--start-of-month: Fixed date options--start-of-week: Fixed date options--start-of-year: Fixed date options--starting-day=argument: Calendar options--suppress-calendar: Calendar options--suppress-date-part: Fixed date options--suppress-fixed-dates-list-separator: Fixed date options--suppress-holiday-list-separator: Calendar options--suppress-text-part: Fixed date options--text-variable=argument: Fixed date options--time-offset=argument: Calendar options--today: Fixed date options--tomorrow: Fixed date options--transform-year=argument: Calendar options--translate-string=text: Calendar options--type=special|standard: Calendar options--usage=?: Common options--usage[=argument]: Common options--version: Invoking gcal2txt--version: Invoking txt2gcal--version: Invoking tcal--version: Common options--week: Fixed date options--with-week-number: Calendar options--year: Fixed date options--zero-dates-only: Fixed date options--zodical-marker-holidays: Calendar options-?: Common options-??: Common options-@dvar[+|-]nwww: Fixed date options-@dvar[[+|-]n]: Fixed date options-@e[+|-]nwww: Fixed date options-@e[[+|-]n]: Fixed date options-@t[+|-]nwww: Fixed date options-@t[[+|-]n]: Fixed date options-A: Fixed date options-a: Fixed date options-b 1: Calendar options-b 12: Calendar options-b 2: Calendar options-b 3: Calendar options-b 4: Calendar options-b 6: Calendar options-b number: Calendar options-C: Fixed date options-c: Fixed date options-c*dn: Fixed date options-c*dnwww: Fixed date options-c*wn[www]: Fixed date options-C-: Fixed date options-c-: Fixed date options-c@dvar[+|-]nwww: Fixed date options-c@dvar[[+|-]n]: Fixed date options-c@e[+|-]nwww: Fixed date options-c@e[[+|-]n]: Fixed date options-c@t[+|-]nwww: Fixed date options-c@t[[+|-]n]: Fixed date options-c[m|M[+|-]]: Fixed date options-c[t|T]: Fixed date options-c[w|W[+|-]]: Fixed date options-c[y|Y[+|-]]: Fixed date options-cA: Fixed date options-ca: Fixed date options-cd: Fixed date options-cE: Fixed date options-ce: Fixed date options-cg[text]: Fixed date options-cJ: Fixed date options-ck: Fixed date options-cl: Fixed date options-cl*dn: Fixed date options-cl*dnwww: Fixed date options-cl*wn[www]: Fixed date options-cl@dvar[+|-]nwww: Fixed date options-cl@dvar[[+|-]n]: Fixed date options-cl@e[+|-]nwww: Fixed date options-cl@e[[+|-]n]: Fixed date options-cl@t[+|-]nwww: Fixed date options-cl@t[[+|-]n]: Fixed date options-clmmdd: Fixed date options-clmmwwwn: Fixed date options-cln+|-: Fixed date options-clnd: Fixed date options-clnw: Fixed date options-cm: Fixed date options-cm+: Fixed date options-cm-: Fixed date options-cmmdd: Fixed date options-cmmwwwn: Fixed date options-cn+|-: Fixed date options-cnd: Fixed date options-cnw: Fixed date options-co: Fixed date options-cQ: Fixed date options-cT: Fixed date options-ct: Fixed date options-cU: Fixed date options-cw: Fixed date options-cw+: Fixed date options-cw-: Fixed date options-cx: Fixed date options-cy: Fixed date options-cy+: Fixed date options-cy-: Fixed date options-cZ: Fixed date options-cz: Fixed date options-c|C[-]: Fixed date options-d: Fixed date options-D argument: Fixed date options-E: Fixed date options-e: Fixed date options-f|F name[+...]: Fixed date options-G: Calendar options-g[text]: Fixed date options-h: Common options-H no: Global options-H text: Global options-H yes: Global options-hh: Common options-i: Calendar options-I pattern: Fixed date options-i[-]: Calendar options-J: Fixed date options-j: Calendar options-j[b]: Calendar options-jb: Calendar options-jc: Calendar options-jc[b]: Calendar options-jcb: Calendar options-jn: Calendar options-jn[b]: Calendar options-jnb: Calendar options-k: Fixed date options-K: Calendar options-l: Fixed date options-L: Common options-l*dn: Fixed date options-l*dnwww: Fixed date options-l*wn[www]: Fixed date options-l@dvar[+|-]nwww: Fixed date options-l@dvar[[+|-]n]: Fixed date options-l@e[+|-]nwww: Fixed date options-l@e[[+|-]n]: Fixed date options-l@t[+|-]nwww: Fixed date options-l@t[[+|-]n]: Fixed date options-lmmdd: Fixed date options-lmmwwwn: Fixed date options-ln+|-: Fixed date options-lnd: Fixed date options-lnw: Fixed date options-m: Fixed date options-M: Fixed date options-m: Fixed date options-m+: Fixed date options-m-: Fixed date options-M[+|-]]: Fixed date options-m[+|-]]: Fixed date options-m|M[+|-]]: Fixed date options-N: Calendar options-n: Calendar options-N-: Calendar options-n-: Calendar options-n|N[-]: Calendar options-o: Fixed date options-O: Calendar options-p: Global options-P argument: Fixed date options-Q: Fixed date options-q cc[_tt][+...]: Calendar options-r argument: Fixed date options-R name: Global options-s argument: Calendar options-S name: Global options-T: Fixed date options-t: Fixed date options-U: Fixed date options-u: Calendar options-V: Common options-v argument: Fixed date options-w: Fixed date options-W: Fixed date options-w: Fixed date options-w+: Fixed date options-w-: Fixed date options-W[+|-]]: Fixed date options-w[+|-]]: Fixed date options-w|W[+|-]]: Fixed date options-X: Calendar options-y: Fixed date options-Y: Fixed date options-y: Fixed date options-y+: Fixed date options-y-: Fixed date options-Y[+|-]]: Fixed date options-y[+|-]]: Fixed date options-y|Y[+|-]]: Fixed date options-Z: Fixed date options-z: Fixed date options@file: Response fileCommand, .: 3-Month mode commandsCommand, . yyyy: 3-Month mode commandsCommand, .+: 3-Month mode commandsCommand, .+ yyyy: 3-Month mode commandsCommand, .-: 3-Month mode commandsCommand, .- yyyy: 3-Month mode commandsCommand, ..: 3-Month mode commandsCommand, .. yyyy: 3-Month mode commandsCommand, :: Single commandsCommand, :yyyy: Single commandsCommand, mm yyyy: Single commandsCommand, mm yyyy-yyyy: Ranges of commandsCommand, mm,...,mm: Lists of commandsCommand, mm,...,mm yyyy: Lists of commandsCommand, mm,mm/yyyy,...: Lists of commandsCommand, mm-mm: Ranges of commandsCommand, mm-mm yyyy: Ranges of commandsCommand, mm-mm yyyy+yyyy: Ranges of commandsCommand, mm/yyyy: Single commandsCommand, mm/yyyy,...,mm/yyyy: Lists of commandsCommand, mm/yyyy-mm/yyyy: Ranges of commandsCommand, mm/yyyy-yyyy: Ranges of commandsCommand, mm:: Single commandsCommand, mm:;mm:;...: Lists of commandsCommand, mm:;mm:yyyy;...: Lists of commandsCommand, mm:yyyy: Single commandsCommand, mm:yyyy+mm:yyyy: Ranges of commandsCommand, mm:yyyy;...;mm:yyyy: Lists of commandsCommand, month name: Single commandsCommand, NOTHING: Single commandsCommand, yyyy: Single commandsCommand, yyyy+yyyy: Ranges of commandsCommand, yyyy;...;yyyy: Lists of commands