I'd like to replace my Ultegra 6800 crankset with QUARQ DFour powermeter. But I don't know how to choose between BB30 and GXP versions. I have Shimano Pressfit SM-BB92-41B bottom bracket. Do I need to replace my bottom bracket or use some adapters?
Your Shimano SM-BB92-41B is a BB92 standard bottom bracket.
Quarq's page lists GXP, Press Fit GXP, BB30, Press Fit 30, BBright, BB386EVO, BB30A, PF30A as compatible bottom brackets.
I believe the Press Fit GXP from SRAM is a BB92 standard bottom bracket. See https://www.parktool.com/blog/repair-help/bottom-bracket-standards-1#article-section-4.
So the GXP version is the one you want. It would be nice if Quarq provided a better compatibility guide.