I'm not sure how reliable my memory is on these matters, but here's a list of what I recall doing in GMA.
Vectors, ellipses/hyperbolas, complex numbers, sequences and series, circle geometry, partial fractions, trigonometry (sine and cosine rules), calculus (differentiation of sine, cosine, tangent, chain and product rules; yes my school did this in GMA year 11)...I don't think I recall anything else that my class did. My class didn't do dynamics in year 11 from memory.
Out of these, the calculus is covered in Methods 3/4, the circle geometry is almost not needed, the sequences and series is definitely not needed, all of the first three topics ARE needed and are not covered in Methods, and trigonometry is assumed knowledge in spesh and doesn't often appear on exams.
GMA (I detest the label 1/2 spesh because that technically doesn't exist as a subject; GMA doesn't technically either according to the VCAA syllabus as the year 11 subject is just called General Maths, but at least books call it Advanced General Maths

) is useful for spesh, but it's not essential. Granted, you'll need to do a bit of work by yourself but it's not impossible. Heck, I learnt the spesh course before I learnt the GMA course

I doubt you can't do it.
As for whether physics helps with GMA...I did spesh before I learnt any real physics too
