It’s not just marketing. Gravel wheel sets have wider internal rim width and are e. g. tubeless-ready or tubeless-only. For example, 3T’s aero gravel wheel set has an internal width of 29 mm. They are also sturdier. Some bikes allow you to mount 27.5/650b mtb wheels with even wider tires.
Yes, you can use narrower rims for wider tires. But one way or another I wouldn’t use carbon rim brake wheels.
And I know a guy who used to use an old Colnago with cantis (2009-2011ish if memory serves) for a while as a gravel bike. He had old pets lying around and didn’t want to invest a fortune to dip his toes into the water. He’s since upgraded to a disc brake bike.