gnus-default-article-saver

gnus-default-article-saver is a variable defined in `gnus-art.el'.
Its value is

gnus-summary-save-in-rmail

Documentation:
A function to save articles in your favorite format.
The function will be called by way of the `gnus-summary-save-article'
command, and friends such as `gnus-summary-save-article-rmail'.

Gnus provides the following functions:

* gnus-summary-save-in-rmail (Rmail format)
* gnus-summary-save-in-mail (Unix mail format)
* gnus-summary-save-in-folder (MH folder)
* gnus-summary-save-in-file (article format)
* gnus-summary-save-body-in-file (article body)
* gnus-summary-save-in-vm (use VM's folder format)
* gnus-summary-write-to-file (article format -- overwrite)
* gnus-summary-write-body-to-file (article body -- overwrite)
* gnus-summary-save-in-pipe (article format)

The symbol of each function may have the following properties:

* :decode
The value non-nil means save decoded articles. This is meaningful
only with `gnus-summary-save-in-file', `gnus-summary-save-body-in-file',
`gnus-summary-write-to-file', `gnus-summary-write-body-to-file', and
`gnus-summary-save-in-pipe'.

* :function
The value specifies an alternative function which appends, not
overwrites, articles to a file. This implies that when saving many
articles at a time, `gnus-prompt-before-saving' is bound to t and all
articles are saved in a single file. This is meaningful only with
`gnus-summary-write-to-file' and `gnus-summary-write-body-to-file'.

* :headers
The value specifies the symbol of a variable of which the value
specifies headers to be saved. If it is omitted,
`gnus-save-all-headers' and `gnus-saved-headers' control what
headers should be saved.

You can customize this variable.