According to research done for Bicycle Retailer & Industry News by 99spokes the median width for gravel tires has grown to 40mm. That’s a sizeable jump from 5 years ago when 32mm was the standard.
Obviously, this doesn’t say a whole lot about the right width for you. The perfect tire depends on where you live, how you ride, what events you want to participate in, weather conditions and so on. You might even swap tires or wheelset/tire combos by event.
Personally, I hate driving somewhere to ride. So, I have to ride 20 miles or more just to get to gravel. So I chose a 38mm tire with a slick center track. I’m still fast on the tarmac and I can turn without sliding on the gravel. There is no perfect gravel tire, but the Schwalbe G-One Allround has been serving me well for what I ride.