gnus-group-mode-line-format

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

"Gnus: %%b {%M%:%S}"

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

%S The native news server.
%M The native select method.
%: ":" if %S isn't "".

You can customize this variable.