Question: are gxp and hollowtech compatible?
Answer: Hollowtech II cranks use also 24 mm cranks, but the GXP ones are thinner (22 mm) on the other end. It is not compatible and you need a Hollowtech II compatible press-fit BB (e.g. BB-MT800 press fit) compatible with your frame (the housing dimensions, the outer BB diameter + the housing length).