Re: gnus-message-archive-group does not work when I compose-mail

After searching in the emacs documentation, and thanks to the
"apropos-documentation" command and searching for "GCC", I found that I
needed to set (setq mail-user-agent 'gnus-user-agent).

> mail-user-agent is a variable defined in ‘simple.el’.
> 
> Its value is ‘gnus-user-agent’
> Original value was 
> ‘message-user-agent’
> 
> Your preference for a mail composition package.
> Various Emacs Lisp packages (e.g. Reporter) require you to compose an
> outgoing email message.  This variable lets you specify which
> mail-sending package you prefer.
> 
> Valid values include:
> 
>   ‘message-user-agent’  -- use the Message package.
>                            See Info node ‘(message)’.
>   ‘sendmail-user-agent’ -- use the Mail package.
>                            See Info node ‘(emacs)Sending Mail’.
>   ‘mh-e-user-agent’     -- use the Emacs interface to the MH mail system.
>                            See Info node ‘(mh-e)’.
>   ‘gnus-user-agent’     -- like ‘message-user-agent’, but with Gnus
>                            paraphernalia if Gnus is running, particularly
>                            the Gcc: header for archiving.