I rode a cyclocross bike with canti brakes as a "gravel bike" for about 10 years before they invented "gravel bikes". That bike can only handle a ~35mm or so tire easily. I found that tire satisfactory, for all of the non-paved riding I do, and have a fairly new pair, so I moved them over to my new disc brake GR300. That's how I've picked my gravel tires. I'll probably try a 38- 40 or so when i need new tires. The frame will take ~45mm with 700c wheels and ~2.1" with 650B. I might end up with ~45 or so tires on a set of 650B and keep ~38's on the 700c. I don't see a need for really big tires on a gravel bike. i don't use it for technical or overly rough stuff, that's what my mountain bike is for. I don't use it for dedicated road rides, that's what my road bike is for.
if I had one bike (which is the case when we're traveling... using the above CX bike), I would have two sets of wheels: 35-38mm for non-paved and general use, and 25-28 for dedicated pavement riding when I want to ride in a road group. That's proved to be very useful for the several years we've been traveling that way.