Like our older ( 30's - 60's) bike conversions, the gravel bike can be developed in a number of ways. The '70s Schwinn road frames that came with 27" wheels, can easily handle a 650b or 700c wheel with knobby rubber. Or like Capt said, many use the '80s - early '90s (steel framed, aluminum frames don't allow as much room in the chain stay / BB area) as a base and add a 650b or 700c wheelset, with narrower knobby rubber than a traditional 1.95 - 2.2 26er tire.
The modern / currently made gravel bikes have more road bike geometry, with usually taller head tubes for more 'stack height' and a higher hand position, which helps when the gravel gets heavy and loose or soupy and muddy. Keeps the weight off the front better to 'float' through those situations. But the reach, essential top tube length, tighter rear triangle i.e. chain stay length are more road bike than hybrid.
Another feature is the drop bar itself. Much wider than the old vintage road bike, and often 'splayed out' in the drops to make for a shallower drop, better handling and maneuverability once the going gets rough. The young and / or skilled guys will ride these on tamer, 'flowy' off-road trails as well.
I turned a mid-'90s Trek 730 Multi-track hybrid into a gravel style road bike a couple of years ago in Arizona. I ride the roads, canal bike paths, and the many connecting desert dirt trails that run in the brush on the way to heavier duty off-road trails. It's an all-roads bike, without suspension, doesn't look like a 'mountain bike', and has that great ride of a steel frame. "Steel is real". It isn't as quick accelerating or as widely geared or nearly as comfortable and light weight as my Trek Checkpoint ALR 4 gravel bike here at home; but it works, and it's 'built not bought'.
Here's a couple photos of the '90s Trek 730 conversion.
Drop bar, knobby tired, Trek 730 rat bike build. Combined two '90s bikes; the Trek 730 MultiTrack and a Nishiki road bike with 105 8 spd components. Added a Nitto 45mm 105 drop handlebar, WTB Riddler 700x45C tires, Selle San Marco vintage style saddle, 11-32t cassette , converted 42 - 39t crankset, Kool Stop salmon brake pads on original cantis. Shimano XT SPD pedals. That True Temper OX Made in the USA frame rides like a dream.
And one of the Trek Checkpoint ALR 4 for comparison...
I'll be interested in seeing how you develop your 'gravel grinder'
@GuitarlCarl , should you decide to go this route. I grew up riding my Schwinn Varsity, my first geared bike, on gravel roads in Southern Minnesota. I feel like I've come complete circle now. A very versatile bicycle!
