First sit means first sit, but there are plenty of universities that don't mind if it's not on first sit.
GAMSAT is really only for grad programmes, or if it's been a number of years (typically 5+) since you last undertook any sort of study, and I think that's only like 1 university. 2 gaps years, especially if you spent one of them studying, won't be an issue. It will be clear on the university website if that's a problem.
Plenty of graduate programmes even still just use the UCAT or BMAT.