In the classic era every bike was a gravel bike. The road turned to gravel and you kept racing. Or riding. Same bike was used for Paris-Roubaix and for Paris-Tours.
This might mean that a 10 or 20 year old bike is not classic enough. Go back 50 years and most bikes will take 32mm tires and still have a centimeter of clearance all around for mud and rocks to pass through. Youngest bike in my collection is 59 years old. It rides very well on gravel.