gnus-check-new-newsgroups

gnus-check-new-newsgroups is a variable defined in `gnus-start.el'.
Its value is

ask-server

Documentation:
*Non-nil means that Gnus will run `gnus-find-new-newsgroups' at startup.
This normally finds new newsgroups by comparing the active groups the
servers have already reported with those Gnus already knows, either alive
or killed.

When any of the following are true, `gnus-find-new-newsgroups' will instead
ask the servers (primary, secondary, and archive servers) to list new
groups since the last time it checked:
1. This variable is `ask-server'.
2. This variable is a list of select methods (see below).
3. Option `gnus-read-active-file' is nil or `some'.
4. A prefix argument is given to `gnus-find-new-newsgroups' interactively.

Thus, if this variable is `ask-server' or a list of select methods or
`gnus-read-active-file' is nil or `some', then the killed list is no
longer necessary, so you could safely set `gnus-save-killed-list' to nil.

This variable can be a list of select methods which Gnus will query with
the `ask-server' method in addition to the primary, secondary, and archive
servers.

E.g.:
(setq gnus-check-new-newsgroups
'((nntp "some.server") (nntp "other.server")))

If this variable is nil, then you have to tell Gnus explicitly to
check for new newsgroups with M-x gnus-find-new-newsgroups.

You can customize this variable.