Looking to swap some cranks over and they are GXP whilst the BB is a Hollowtech II one.. will GXP bearings (which I already have) fit in the HTII cups to basically turn them into a GXP BB?

I think that GXP spindles have a smaller diameter on the non-drive side so need a different size reducer to the plastic one in the shimano NDS cup. As far as I know the bearings themselves are the same so should be swappable. It all comes down to the reducers, so if you can save them from your GXP BB then you should be fine.

(this is from memory so may be wrong, I don’t have a GXP to hand)

Cheers, that’s kinda what I assumed..