Scanning New Messages

g: Check the server(s) for new articles. If the numerical prefix is used, this command will check only groups of level arg and lower (gnus-group-get-new-news). If given a non-numerical prefix, this command will force a total re-reading of the active file(s) from the back end(s).
M-g: Check whether new articles have arrived in the current group (gnus-group-get-new-news-this-group). gnus-goto-next-group-when-activating says whether this command is to move point to the next group or not. It is t by default.
C-c M-g: Activate absolutely all groups (gnus-activate-all-groups).
R: Restart Gnus (gnus-group-restart). This saves the .newsrc file(s), closes the connection to all servers, clears up all run-time Gnus variables, and then starts Gnus all over again.

gnus-get-new-news-hook is run just before checking for new news.

gnus-after-getting-new-news-hook is run after checking for new news.