I like a 45-50mm tire that is supple. Small knobs are okay, but a supple smooth tire works fine on packed gravel and pavement too.
Where gravel gets tricky is when there is a hard packed track or two with progressively more loose gavel to the center and sides and often approaching a short bridge or other concrete section. Running off into the loose stuff can dump you if you aren't prepared.
I'm running WTB Horizon 650x47mm tires now and have a pair of WTB Resolute 650x42mm waiting for a 100+ mile gravel rail trail trip this summer.