I really think it depends on what you use the bike for.

For some, gravel is basically road riding we ty a dusty diversion here and there. I think a lot of folks rode these conditions with 23s and 25s for years without a second thought. If that is your gravel, the but we should be geared like a road bike.

For others, myself included, gravel means dirt mountain roads. This includes washboards, sand, ruts, roots, and plenty of sections of mountain bike trails. 11-13mph is a pretty good average speed. 20% grades are always welcome. For the other stuff, I have a nice road bike. I ride 40nw with 11-42. Once on flat roads or pavement, I can comfortably cook along at 17-18 mph and with a tailwind, I can keep pedaling up to 27. Seems like enough for most people.