OP:
sure.
If your gender is female, and you like girls, then you probably are a lesbian.
If you like guys, and your gender is male, then you're probably gay.
If you like both regardless, then you are bi, most likely.
As far as organizations or memberships are concerned?
I think "trans" overrides "gay" and "lesbian"
I wouldn't refer to myself as "gay" or "lesbian". Not when I am already use Trans___________, or when the "T" is already apart of GLBTQ, and the community at large.
No need to double label.
If a friend or someone asks me what sex I prefer for physical contact and lovemaking I would say mostly girls, and some guys. I refer to myself as Bi. If I liked guys exclusively, I would just say I was "into guys" and not use the "gay" label as "trans" overrides that in my opinion, and is more explanitory. Sexual preference is a sub issue compared to that. Using "gay" or "lesbian" when you are transgendered is probably only going to lead to confusion, something I personally wish to avoid.