Good question! The difference primarily comes down to gearing, wide tire clearance, bottom bracket drop, and cable routing will also tend to differ.
A cyclocross bike has a narrower gear range than a gravel bike. Other than the bikes of the fastest European pros (who need a higher gear for the opening sprint to the holeshot), a 1X drivetrain with a 40T chainring or so in the front coupled to an 11-32 cassette in the back covers it for most cyclocross courses. In cyclocross, the really steep hills are short and often too steep or slippery to ride anyway; a lower gear than 40×32 would add weight but not speed. A gear higher than 40 X 11 is rarely needed, as the tight turns and high tire rolling resistance rarely allow building any more speed than that.
By contrast, a gravel bike will be expected to grind up long climbs, so a low gear close to one-to-one will often come in handy. And, a gravel bike will often find itself on long, fast, paved descents, so a much higher gear of at least 50×11 is needed. Look for gears on a gravel bike of 50/34 in front and 11-34 or 11-36 in the rear.
Another detail difference may be tire clearance. A cyclocross bike is limited by UCI rules to a maximum tire size of 700x33C, and a C-shaped measuring tool is used in UCI races to ensure that no tire in the race is wider than 33mm. While abundant mud clearance around the tires is a must in cyclocross, that can also be required on a gravel bike. The gravel bike, however, must have considerably more tire clearance than a cyclocross bike, as it would be expected to fit at least up to 700x42C and likely 700x45C tires (and provide mud clearance around them).
A few other details often differ as well. A gravel bike’s tires might be 10mm taller than those on a cyclocross bike, yet cornering clearance requirements are similar. So, in order to have the bottom bracket height of the aired-up bikes end up being close to the same, the gravel-bike frame will generally have around 10mm more bottom bracket drop (the vertical distance between the center of the bottom bracket and the line connecting the centers of the front and rear hubs) than will the cyclocross frame.
Cable routing for a front derailleur might be lacking on a cyclocross frame, whereas it would likely be present on a gravel-bike frame.
from velonews.com