If you ride a GRX 1x drivetrain back to back with a Deore 12s drivetrain, the mtb groupset is a decade ahead in shifting and durability. SRAM has decent gravel drivetrains, but they are all high-end wireless options.
But that set shifting and durability aside, from a frame-building perspective, gravel vs MTB all comes down to drivetrain and tire clearances.
On page 22 of the SRAM Frame Fit Specifications, you can find the mountain bike chainring/crank clearances:
I laid out all the gravel and road drivetrains here:
With a gravel drivetrain, 430 chainstays, and a 650x50, there is still plenty of clearance. Most pre-bent chainstays will work, as well as custom bent 3/4in chainstays. See the example below:
The only thing I would be concerned about is mixing and matching:
- not all gravel wheels can take microspline freehub bodies
- using mtb frame standards and road/gravel fork hub spacing will make your wheel selection a nightmare
- If you use road/gravel shifters, you should probably make a flamount frame