GXP and Shimano differ in spindle diameter (diameter is perpendicular to the aforementioned spindle length). GXP is 24 mm DS/ 22 mm NDS; Hollowtech is 24 mm DS and NDS. so the two standards are not compatible. See picture below and following website

Shimano


Sram

For the same BB standard, the spindle on MTB cranks is longer than what you find on the corresponding road bike cranks, which means that mtb cranks have a higher Q-factor than corresponding road crank. note that spindle length always measured perpendicular to the crank arm. If you don't mind the higher Q-factor, you can always run MTB cranks on a road bike, but not vice versa (the spindle cannot stick sufficiently far out of the NDS BB cup for one to attach the NDS crank arm)

i use a stages x9 Left crankarm on cross bike (sram red gxp) and MTB (xx1, xx , x9 cranks)

Your diagram for the SRAM crank axle isn't correct.

You are saying that the diameter of the axle is 24mm on both ends and the diameter of the splines is 22mm.

The diameter of the axle on the drive side is 24mm and the diameter of the axle on the non-drive side is 22mm.

Thanks!

It sounds like the right side MTB crank arm will fit on the road spindle, but I believe MTB cranks have more "splay" (wider BB, and even wider Q-factor), so the Q-factor will be wider and not centered under the seat tube.