hiya ive done the sat 3 times and I've always struggled w/ the maths even tho i did cie igcse maths and ib maths. i think the maths is not difficult, it's really about the exam technique and question style. you'll see what i mean as you start doing the past papers. i think you definitely need the maths knowledge from gcse but not so far as the advanced stuff in a levels (i dont do a levels so I'm not too sure whats in the syllabus but there's nothing like calculus or hard trig etc). it's definitely more about time management and technique and seeing the shortcuts in the questions 100000%! not rlly abt how good you are at mastering concepts in gcse level or a level maths. if you'd like, you can start on khan academy to see how the Qs are (i personally think it's better for English prep than maths but it's a great resource nevertheless)