How to set stuff like From, Organization, Reply-To, signature...?
There are other ways, but you should use posting styles for this. (See below why). This example should make the syntax clear:
(setq gnus-posting-styles '((".*" (name "Frank Schmitt") (address "firstname.lastname@example.org") (organization "Hamme net, kren mer och nimmi") (signature-file "~/.signature") ("X-SampleHeader" "foobar") (eval (setq some-variable "Foo bar")))))
The ".*" means that this settings are the default ones
(see below), valid values for the first element of the
following lists are signature, signature-file,
organization, address, name or body. The attribute name
can also be a string. In that case, this will be used as
a header name, and the value will be inserted in the
headers of the article; if the value is
nil, the header
name will be removed. You can also say (eval (foo bar)),
then the function foo will be evaluated with argument bar
and the result will be thrown away.