If you'd like to read the parent of the current article, and it is not displayed in the summary buffer, you might still be able to. That is, if the current group is fetched by NNTP, the parent hasn't expired and the References in the current article are not mangled, you can just press ^ or A r (gnus-summary-refer-parent-article). If everything goes well, you'll get the parent. If the parent is already displayed in the summary buffer, point will just move to this article.

If given a positive numerical prefix, fetch that many articles back into the ancestry. If given a negative numerical prefix, fetch just that ancestor. So if you say 3 ^, Gnus will fetch the parent, the grandparent and the grandgrandparent of the current article. If you say -3 ^, Gnus will only fetch the grandgrandparent of the current article.

Fetch all articles mentioned in the References header of the article (gnus-summary-refer-references).

Display the full thread where the current article appears (gnus-summary-refer-thread). This command has to fetch all the headers in the current group to work, so it usually takes a while. If you do it often, you may consider setting gnus-fetch-old-headers to invisible (see Filling In Threads). This won't have any visible effects normally, but it'll make this command work a whole lot faster. Of course, it'll make group entry somewhat slow.

The gnus-refer-thread-limit variable says how many old (i. e., articles before the first displayed in the current group) headers to fetch when doing this command. The default is 200. If t, all the available headers will be fetched. This variable can be overridden by giving the A T command a numerical prefix.

You can also ask the NNTP server for an arbitrary article, no matter what group it belongs to. M-^ (gnus-summary-refer-article) will ask you for a Message-ID, which is one of those long, hard-to-read thingies that look something like `<38o6up$6f2@hymir.ifi.uio.no>'. You have to get it all exactly right. No fuzzy searches, I'm afraid.