I think the best gravel bike is a XC hardtail with a rigid fork.

The top "gravel" tires over at bicycling rolling resistance are a pair of treadless XC tires from continental.

I guarantee you a 2.2 inch 700 c tire will be faster on gravel. It may not feel like it in the first 1 min, but after 1 hour...it'll be faster.

Watch: In ten years, gravel bikes will be running 2.2 tires.

The best XC rider in the world is running basically treadles 2.4 tires.