gnus-summary-mode-line-format

gnus-summary-mode-line-format is a variable defined in `gnus-sum.el'.
Its value is

"Gnus: %g [%A] %Z"

Documentation:
*The format specification for the summary mode line.
It works along the same lines as a normal formatting string,
with some simple extensions:

%G Group name
%p Unprefixed group name
%A Current article number
%z Current article score
%V Gnus version
%U Number of unread articles in the group
%e Number of unselected articles in the group
%Z A string with unread/unselected article counts
%g Shortish group name
%S Subject of the current article
%u User-defined spec
%s Current score file name
%d Number of dormant articles
%r Number of articles that have been marked as read in this session
%E Number of articles expunged by the score files

You can customize this variable.