My Eisentraut Limited currently sports 32mm smooth tread Gravel Kings. I've ridden it on the Des Plaines River Trail a couple times this year with no particular problem (one tumble, but I chalk that up to pilot error). I think I could fit 35s with a more aggressive tread. That's probably all I need. In general, I think most production road bikes made before the late 70s probably have clearance for 32s, if not wider.
Maybe there's a difference I'm missing, but are modern gravel bikes all that different than modern cyclocross bikes?