There's a huge difference between A level Maths (and by extension FM) and GCSE maths. I learnt what I needed for all the GCSE Maths exam in year 9, and to be brutally honest, there isn't nothing hard in it if you're competent at Maths. A-level maths is a completely different, and while I don't find it difficult so far, it's a step up from GCSE Maths.
If you can, try and do FSMQ. It pretty much covers all of C1 and C2 in AS Maths (bar Logs and sequences), and it even throws in a bit of D1 (Linear Programming) and M1 (Kinematics). See how you do in that.
If you can't, who cares? As long as you feel you're good in Maths and you're interested in it, take it.
If you're good at maths, there is no reason why you shouldn't take it. And to be fair, GCSE Maths doesn't mean ****.