Gideon, I always thought, was... well, not homophobic, exactly, but less than comfortable around gay people. When Jeremy announces his new love interest, Jason's expression sort of freezes, as if he's not quite sure what to say. Then he comes out with something neutral like "Well... that's a good reason." Jeremy, of course, then hands him the key to solving the case.

Then in "Somebody's Watching", when the lesbian artist (another nicely written/acted bit part) starts telling him how attractive he is, Jason practically has a panic attack.

The show definitely views gay characters as solidly normal, of course. Even Rossi gets into the act, practically tearing Clueless!Abusive!Dad a new one in "In Heat". I think Gideon is just a product of his (our) generation, and not altogether comfortable with the concept.