for the most part, to paraphrase Steve and others re: science fiction, gay fiction is what we point to when we talk about gay fiction. In publishing terms, gay fiction almost invariably involves some flavor of non-heteronormal author and subject matter. (Man, is it hard to find the right way to phrase that.) Sometimes, but not always, it's aimed specifically at an audience of the same.
Does it HAVE to be that? Eh, I'm not that interested in quibbling over definitions. If someone wants to count work by straight authors about gay characters, or vice versa, or some other atypical scenario as gay fiction, they can knock themselves out. I'll read the description, then read it or not as my tastes dictate.
Sorry, decades of arguing about what constitutes science fiction has left me a little too tired to care about these things that much anymore.