gnus-sorted-header-list

gnus-sorted-header-list is a variable defined in `gnus-art.el'.
Its value is

("^From:" "^Subject:" "^Summary:" "^Keywords:" "^Newsgroups:" "^Followup-To:" "^To:" "^Cc:" "^Date:" "^Organization:")

Documentation:
*This variable is a list of regular expressions.
If it is non-nil, headers that match the regular expressions will
be placed first in the article buffer in the sequence specified by
this list.

You can customize this variable.