just to add further confusion, some 11sp road cassettes are the same width as 11sp MTN cassetes and also require a spacer on 11sp road wheel but will of course work with MTN hubs. 🙂
Gravel wheels are generally slightly wider (internally) than road wheels but can run higher pressures than 29er MTB wheels which are radialy the same dimension as road and gravel wheels at 622mm.
Personally, for gravel, I use XC 29er rims on road hubs and spokes. this allows for a nice light wheel, but precludes the possibility of using anything less than 30mm tyres as the rims will only support a max pressure of 75psi. For me this isn’t a problem as I use 32mm tubeless road tyes and 40mm gravel tyres. My lightest wheelset is 1408g on not particularly light Chris King hubs.