I'm a gay guy. I've spent a lot of time in both straight and gay clubs - including regular clubs, raves, trance parties, psytrance parties, outdoor parties, after hours and other special music events. Drug availability is about the particular scene and venue, not sexual orientation.
The only time I think it benefits being in a gay environment to score is related to age. I'm 36 and in a lot of straight clubs I feel like a grandpa and youngsters would look at me like a total perv or narc if I tried to sniff around. Gay clubs certainly vary by age, but there are many gay dance clubs where there are a wide variety of people from barely legal to having grey or white hair out there tearing it up on the dance floor. With such a wide age variety there's a much greater chance that there is a wide range of connections for whatever you need or want.
By the way, I wish to soundly refute the poster who said that gay people use more drugs than straight people. At the very least, you need to post a study that validates that claim. In my circle of gay friends there are just as many people who hate drugs and have never tried them as people who have tried them on a range from once to being total drug heads.