gnus-post-method

gnus-post-method is a variable defined in `gnus-msg.el'.
Its value is

current

Documentation:
*Preferred method for posting USENET news.

If this variable is `current' (which is the default), Gnus will use
the "current" select method when posting. If it is `native', Gnus
will use the native select method when posting.

This method will not be used in mail groups and the like, only in
"real" newsgroups.

If not `native' nor `current', the value must be a valid method as discussed
in the documentation of `gnus-select-method'. It can also be a list of
methods. If that is the case, the user will be queried for what select
method to use when posting.

You can customize this variable.