@tk@biketooter@cycling@mastobikes imho the point of a gravel bike is not to be more/as efficient as a mountain bike off the road but to bring the feeling of a road bike while having dirt roads capabilites. I am saying this as someone who did MTB marathon events on a cyclocross bike back when gravel bikes weren't already a word.
For that reason, even if it might be slower I prefer riding a gravel bike with 650b wheels as the outer diameter feels closer to a 700c wheel with 28mm tire. The bike might not be as fast asa a gravel bike with 700x 45mm tires but I don't care, especially as I am not racing any more. 650B feels more playful and lively.
If I want better off road capabilities, I just ride my 27.5fat or 29+ mountain bike.