I’m aware that the axles on shimano MTB cranks are different in length to those on the road ones (by virtue of BB shell sizing).

I have a GRX810 crank. I also, on my XC MTB have a stages left hand XTR crank arm. Both are the same length

Will this fit on the end of my GRX driveside crank?

It depends on the axle spline of the GRX crank – on my Mason ISO I have GRX 810 but an XT 8000 crank with a 4iiii PM.

I think the xt cranks stick out more to give greater frame/tyre clearance, so if you are sensitive about such things you might feel a bit lopsided, I suspect most people wouldn’t notice. I saw on the gplama youtube channel doing it the other way round (road PM crank on gravel bike) and the clearance was very tight.

It’s a pretty simple thing to try out anyway.

As said above the Q factor (how wide the pedals are spaced) is different.but from memory it’s the axel width that is different (less spacers in the BB than MTB) so you may be fine….