Seeing that it's Year 11, it's probably not that bad of a mistake. There's nothing wrong with trying out subjects for the sake of it.
Likewise, you don't need Unit 1/2 Specialist (or General Maths Advanced or whatever they call it) for 3/4 Specialist. The stuff that isn't covered in Methods is all explained from scratch and if you're willing to put the effort in, you can get your head around it easily enough.
Seeing that you decided to move into the subject, I assume you're decently interested in it. If you really want to do specialist, I would stick with it for at least Unit 2 and see what you think of it. I don't know what's exactly covered in Unit 2 GMA as I didn't do it - but if 3/4 Spesh is easy enough to pick up, Unit 2 Spesh shouldn't be too bad (I'd assume you'd have to be prepared to learn a lot of new concepts in one go however, particularly new stuff that was taught in Unit 1).
But yeah, if you really don't like Unit 2 GMA, you always have the option of picking up Further 3/4 for next year too.