Yes, Gnus is ex(c)iting.
Suspend Gnus (gnus-group-suspend). This doesn’t really exit Gnus,
but it kills all buffers except the Group buffer. I’m not sure why this
is a gain, but then who am I to judge?
Quit Gnus (gnus-group-exit).
Quit Gnus without saving the .newsrc files (gnus-group-quit).
The dribble file will be saved, though (see Auto Save).
gnus-suspend-gnus-hook is called when you suspend Gnus and
gnus-exit-gnus-hook is called when you quit Gnus, while
gnus-after-exiting-gnus-hook is called as the final item when
exiting Gnus.