For a given page in Geminispace, backlinks are all the other pages in Geminispace that link to that page. When viewing GUS search results in verbose mode, a link to view each result's backlinks, if there any, will be provided.
The URL structure for retrieving a certain URL's backlinks page is predictable, should you want to link directly to it in other contexts. All you need to do is URL encode the entire URL you want information on, then pass that as a query to gemini://gus.guru/backlinks. An example follows:
gemini://geminispace.info/backlinks?geminispace.info
Note the distinction between "internal" and "cross-capsule" backlinks. Internal backlinks are backlinks from within your own capsule to the given page. Cross-capsule backlinks are backlinks from other users' capsules. Note that the cross-capsule determination is slightly more advanced than purely checking if the hosts are different - it also takes into account different users on pubnixes, so, for example, gemini://foo.bar, gemini://foo.bar/~ronald, and gemini://foo.bar/~mcdonald would all be considered distinct capsules, as they are all presumably authored and maintained by distinct humans.