Gabriel Osbourne is not a psychic. As Patrick Jane himself would say: "There's no such things as psychics."
It's possible that Gabriel was mentally unstable to the point that he actually believed he was psychic himself (the fact that his sister - a close personal support in his life - also promoted him as psychic surely didn't help.) He was certainly a little bit "off." That said, nothing presented in the course of the show has ever suggested that there are any REAL supernatural powers out there, so there's no reason to start thinking so on account of Gabriel.
As for how Gabriel could have known what he knew; Lisbon suggested two possibilities herself:
1) he might have a friend in the evidence lab (or other menial position in law enforcement.)
2) he might be the killer (this one is later proven false, of course.)
On top of that, there was an earlier episode (S04E03: "Pretty Red Balloon") where Jane himself did the whole "psychically predict where the victim is in the middle of the woods" gag, and he even stopped to explain how he did it. IIRC, Jane talked about how "saying it's near a body of water is a safe guess" because there's almost always a pond or a stream or something in the woods.
Gabriel is an established psychic, for better or for worse, and he probably knows all the same tricks Jane does. How to be just specific enough to get people going while staying vague enough to avoid outing himself. It's what the show is all about.