I've ridden a Pinarello Record (sweet '80s roadie with 26mm tires max) and a KHS Montana (badass 1997 rigid MTB with Michelin Country Rock semi-slicks) for long distances on I-5.

The more gravel involved the more I'd tend towards the KHS.

My old Univega Super Special road bike took 35mm CX knobbies just fine and KILLED on the gravel, so I'd ride that on loose gravel.

I'm wanting to get into gravel grinding races. Most folks run old roadies/CX rigs/MTBs and they seem to be pretty low key. That's my kinda scene.