Basically you want the biggest tires that will fit, but it's not a matter of fitting the wheels -- almost anything will fit almost any rim. The fit issue is with the frame and fork.
If a lot of your ride is on pretty loose and rough stuff, tires with a little tread may help, but it doesn't matter as much as you might think. Cross tires may be a good choice - if they'll fit your frame and fork.
What's your bike, and what rims do you have? Take a look at the clearances with your present tires. How much room is there where the clearance is smallest?
And how much is the load (you plus bike plus whatever you carry)?
Like JCavilia wrote, the biggest tires that will fit. I've been riding Compass road tires and have had good luck on gravel. But the bike is 650b x 42mm and most 700c bikes won't fit a tire that large, but if 32 or 38mm tires would fit your bike, that'd be good.
I just read a blog post about traction on gravel and the use of road tires. The gist of it was that the loss of traction wasn't from the interface of the tire on the gravel, but from the gravel on gravel interface. The point was that the "buoyancy" of a larger diameter tire keeps it on top of the gravel rather than riding thru it, and it's the gravel layers under the tire that are slipping.
In my experience this holds true. Riding gravel, on my tires, is not an issue until the gravel gets deep and loose enough to overpower my tires. The larger the tire, the more float it has to ride on top of the gravel as the gravel gets deeper.
I've found Maxxis refuse to be very nice tires in terms of durability while still riding okay. They aren't going to feel like race tires but they are the most flat resistant road tires I've ever ridden. I the newer tubeless models in the wider widths is nice as well but I imagine that won't fit you're frame as 32mm is the narrowest tubeless option.
Most road tires will work on hardpack gravel, lower the tire pressure some and take it easier in the corners. Avoid chatter bumps and potholes as much as possible and get out of/float over the saddle when going over them. Using the softer race tires aren't the best option as the rubber tends to get cut up a bit and you'll lose some longevity. I personally like the vittoria pave' or similar for gravel racing if I'm not rolling CX tubulars, as example of a higher end tire that does well on gravel.
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