Mix SRAM eTap with GRX Group
It's unclear what you mean by 'changing the shifting system to eTap but stick to the braking system'. You can't separate the two systems.
If you change shifters you are changing the brake levers. Shimano and SRAM hydraulic brake levers and calipers are utterly incompatible as they use different hydraulic fluids (SRAM DOT fluid, Shimano mineral oil).
If you are replacing the shifters/brake levers why not replace the calipers too?
Shimano provides extensive info on all their bicycle products. Start here https://productinfo.shimano.com/#/. Specific GRX810 info is here https://productinfo.shimano.com/#/lc/rx810/mech_2x11.