Gravel is literally 35mm tyres - it's just that in the UK we don't really have a gravel roads network like they do in the states so everyone ends up building something that's better than a rigid 29er off road 1% of the time and worse 99%.
That's why I ride a rigid 29er on my 'gravel' rides. I take the 38mm slick tyred bike sometimes, and feel smug gliding up the road climbs, then get left, swearing at the back on the descents.
Gravel is literally 35mm tyres - it's just that in the UK we don't really have a gravel roads network like they do in the states so everyone ends up building something that's better than a rigid 29er off road 1% of the time and worse 99%.