There is no real gaybourhood. Too many of the locals are still living at home. Gay people are pretty much spread out everywhere.
Having said that, Ximen probably has the highest concentration of residents, but I would personally never live there-the apartments are mostly old and smelly, plus that place is just crammed with teeny-boppers all the time.
You are better off just living somewhere on the blue line where you can easily access all of the gay venues/areas.