Can I replace a GXP press fit bottom bracket with a Hollowtech? I've recently acquired a frame for a project and have an Ultegra crank set from my crash bike I want to use on it.  

The frame currently has a GXP 24mm press fit BB and I've got a spare Hollowtech BB, a Shimano BB-R60 threaded unit. Can I replace the press fit one with the Hollowtech?
You cannot fit a threaded bottom bracket into a press-fit shell frame. A threaded Hollowtech II BB for road cranks will only fit in ISO threaded bottom bracket shell of width 68mm.

GXP is a crank axle standard not a BB standard, so that does not tell us what the press fit cups in the shell are. What you need to do is measure the cup diameter and shell with. Look up what BB shell standard you have (easy to find on the Internet). Then find a BB that adapts between the frame and Uletgra crank.

With any luck you will have a BB86 frame (41mm cups, 86mm wide) - that's the Shimano press-fit standard and sometimes know as Hollowtech press fit (it's the same axle dimensions as a threaded Hollowtech BB). I believe road GXP is also 86mm wide so it seems likely. if you have a shell with larger cups designed to accommodate 30mm axles there are adapters available.