gnus-verbose-backends

gnus-verbose-backends is a variable defined in `nnheader.el'.
Its value is

7

Documentation:
Integer that says how verbose the Gnus backends should be.
The higher the number, the more messages the Gnus backends will flash
to say what it's doing. At zero, the Gnus backends will be totally
mute; at five, they will display most important messages; and at ten,
they will keep on jabbering all the time.

You can customize this variable.