To start Emacs in Gnus:
emacs -f gnus
emacs -f rmail
A more convenient way to start with Gnus:
alias gnus 'emacs -f gnus' gnus
It is probably unwise to automatically start your mail or news reader from your .emacs file. This would cause problems if you needed to run two copies of Emacs at the same time. Also, this would make it difficult for you to start Emacs quickly when you needed to.