Though many of you (myself included) primarily identify as road riders - by fact if not by choice - the increasingly clueless and psychotic drivers on said roads, plus the allure of making a bit of an adventure of it, means that more and more of us are riding dirty.
Of course in order to ride gravel (and I guess we've just come to accept that word), you need to have some nearby. This is one of the nice things about Vermont. Boy does it have unpaved roads. And as cross country ski season fades by the second, bike season dawns. You may notice that this bike pictured above (photo taken on Sunday) is not a "gravel bike," but that's the bike that lives in VT so that's how it gets used this spring. One bike, two sets of wheels. It's a thing.
The tires on this sucker are 40mm Schwalbe G-One Speeds (edit - these are now call G-One All Around, I'm just used to calling the G-One speeds for the last couple of years, and stated size is actually 38), which have a minimal tread. But they're 40mm wide, so they have a big contact patch. The tires I used for a bunch of gravel riding this summer were Donnelly MSO 36s, which have much more tread, though still not what you'd call aggressive, but they're a bit narrower.
Different bikes, but same wheels in both photos - RCG 36s. More on that in a second.
You can of course go with some really aggressive treads as we see on these GX24 wheels, with Maxxis Ikons
The dimensions of the question boil down to three parameters, at least for me: speed, comfort, and grip. I don't know what else you'd want to consider beyond those three. The problem is nudging one or the other up without nudging both of the others down.
CyclingTips had a good article about this, in the context of gravel racing, a while ago. You can find it here. Their basic premise is that percentage of dirt vs paved dictates tire width, and surface type and your skills dictate tread. Since this is racing, comfort isn't a big deal other than being too uncomfortable is slow.
Most of us like to go fast, but comfort isn't a bad thing. Suspension and suppleness equal comfort. Suppleness is baked into the type of tires you buy, but suspension is a mix of volume and pressure. With the 40mm G-Ones, I started off with about 40 psi and quickly backed off a bit from that. This increased comfort, didn't noticeably decrease speed on paved (though the mountain bike, even with suspension locked, is a pretty slow platform until you get onto proper trails), but boy did it increase grip. On steep climbs that had me wishing for more than the 32x26 shortest gear I have (which is most of them), taking the pressure down to 30 virtually eliminated rear wheel slip.
If you're lucky enough to have all trails and aren't worried about speed as much, then the wide 650b tires with more aggressive tread may be your ticket. You can get those suckers PLUSH.
The Donnelly 36s mate up quite well with the 21.5mm internal RCGs, and would also do with Belgium+ or other similar width rims. While the 40mm G-Ones work well with them, I think you might be able to get a skooch more out of them with wider 25mm internal rims a la All Road or Belgium G, just for better support at low pressure. That "almost 40" tire width really seems to be the crossover zone there.
The general takeaway is that unless you're really trekking through mud and muck, you likely don't need a whole ton of tread. A bit of width and the right pressure make a little tread go a long way, and still give you the go-fast. If you do meet the mud, then tires with good knobs that clear mud and don't get packed are going to be the go. You won't be as fast overall, but you'll be much faster and happier in the mud.
Last note, though I don't have either installed in the top photo (the rear fender is an SKS Blade, which works great if you don't have fender eyelets), the Ass Saver/Mucky Nutz combo gives you pretty shocking protection from tire schmutz. They'll do nothing for other people you're riding with, but they do keep a lot of crap off of your body. They don't do much for your feet and lower legs, but for a simple "works on any bike, moments to install and remove" solution, they're awesome.