gnus-saved-headers

gnus-saved-headers is a variable defined in `gnus-art.el'.
Its value is

"^From:\\|^Newsgroups:\\|^Subject:\\|^Date:\\|^Followup-To:\\|^Reply-To:\\|^Organization:\\|^Summary:\\|^Keywords:\\|^To:\\|^[BGF]?Cc:\\|^Posted-To:\\|^Mail-Copies-To:\\|^Mail-Followup-To:\\|^Apparently-To:\\|^Gnus-Warning:\\|^Resent-From:"

Documentation:
Headers to keep if `gnus-save-all-headers' is nil.
If `gnus-save-all-headers' is non-nil, this variable will be ignored.
If that variable is nil, however, all headers that match this regexp
will be kept while the rest will be deleted before saving. This and
`gnus-save-all-headers' will be overridden by the `:headers' property
that the symbol of the saver function, which is specified by
`gnus-default-article-saver', might have.

You can customize this variable.