gnus-visible-headers

gnus-visible-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:
*All headers that do not match this regexp will be hidden.
This variable can also be a list of regexp of headers to remain visible.
If this variable is non-nil, `gnus-ignored-headers' will be ignored.

You can customize this variable.