The following commands are available in the Gnus summary buffer:
If there is no article selected, select the article on the current line and display its article buffer. Otherwise, try scrolling the selected article buffer in its window; on reaching the end of the buffer, select the next unread article.
Thus, you can read through all articles by repeatedly typing SPC.
Scroll the text of the article backwards.
Select the next unread article.
Select the previous unread article.
Do an incremental search on the selected article buffer, as if you switched to the buffer and typed C-s (see Incremental Search).
Search forward for articles containing a match for regexp.
Exit the summary buffer and return to the group buffer.