There is a whole thread devoted to drop bar MTBs any one of which would make a fine gravel bike on the cheap.
https://www.bikeforums.net/classic-v...nversions.html
Cheap and easy? Take a vintage MTB and run a set of trekking bars:
Or you could do a 650b conversion:
https://www.bikeforums.net/classic-v...nversions.html
Or you could as fat a tire as you can stuff in a vintage bike. Most will top out at 35c.
https://www.bikeforums.net/classic-v...tage-bike.html
Or you can find a bike that was designed to be a road bike for bad roads. Some of the bridgestone XO series were designed to be "fire" road bikes, road geometry plus 26 inch tires. Back in the early 90s, 26 inch made a lot of sense if you wanted to run a wider tire. This is my XO 2.
