I believe the option to sit GAMSAT this September may have expired for normal fees so you might have to catch the late registration. There probably isn't enough time to do the GAMSAT well, but if you get the UCAT out the way, you could give the GAMSAT a "trial run" with the aim of applying next year if you don't secure A100 offers this year. You could also risk doing the GAMSAT and applying to GAMSAT unis, but you'd apply blindly without knowing your results, so it's a risk you'd have to weigh up.