Is it possible to be gay-asexual and pansexual at the same time?

You can't have more than one sexual orientation at the same time. Maybe what you're thinking about is the split attraction model: since sexual and romantic orientation can be different, you could be attracted to one or more genders sexually (for example you could be homosexual) and one or more genders romantically (for example biromantic), and the two orientations don't necessarily have to correspond. At the same time, if you experience at least some degree of sexual attraction, asexuality doesn't sound like it's your case. My suggestion is to think of the ways in which you are attracted to people: is it a physical, sexual attraction? Is it more emotional-intellectual and romantic? Is it both? Once you've figured it out, you can determine what your sexual and romantic orientations are, and whether they coincide or not.