gnus-auto-select-next

gnus-auto-select-next is a variable defined in `gnus-sum.el'.
Its value is

t

Documentation:
*If non-nil, offer to go to the next group from the end of the previous.
If the value is t and the next newsgroup is empty, Gnus will exit
summary mode and go back to group mode. If the value is neither nil
nor t, Gnus will select the following unread newsgroup. In
particular, if the value is the symbol `quietly', the next unread
newsgroup will be selected without any confirmation, and if it is
`almost-quietly', the next group will be selected without any
confirmation if you are located on the last article in the group.
Finally, if this variable is `slightly-quietly', the `Z n' command
will go to the next group without confirmation.

You can customize this variable.