Some variables relevant for moving and selecting articles:
gnus-auto-extend-newsgroup ¶All the movement commands will try to go to the previous (or next)
article, even if that article isn’t displayed in the Summary buffer if
this variable is non-nil. Gnus will then fetch the article from
the server and display it in the article buffer.
gnus-select-article-hook ¶This hook is called whenever an article is selected. The default is
nil. If you would like each article to be saved in the Agent as
you read it, putting gnus-agent-fetch-selected-article on this
hook will do so.
gnus-mark-article-hook ¶This hook is called whenever an article is selected. It is intended to
be used for marking articles as read. The default value is
gnus-summary-mark-read-and-unread-as-read, and will change the
mark of almost any article you read to gnus-read-mark. The only
articles not affected by this function are ticked, dormant, and
expirable articles. If you’d instead like to just have unread articles
marked as read, you can use gnus-summary-mark-unread-as-read
instead. It will leave marks like gnus-low-score-mark,
gnus-del-mark (and so on) alone.