Gay is used more generally as a less clinical term than 'homosexual'. So yes, girls can be gay. I personally will describe myself as either 'gay' or 'a lesbian' but don't use lesbian as an adjective (it sounds funny to me, but I'm not sure why!).

My general feeling is that gay people have been called so many awful things throughout history that we should certainly be able to choose our own descriptors now and use them as we choose. But pragmatically, there are other things that influence my choice e.g. the feeling that saying "I'm gay" is les inflammatory, or less of a statement, than saying "I'm a lesbian", and will make others relatively less uncomfortable.

Argh, even ten years since I started, I find the whole thing quite exhausting. I find it easiest of all to come out by saying 'My partner.....' and then 'Yes, she.........". And it's done