What separates a gravel bike from a road or cyclocross bike
The shortest answer is tyre clearance. A modern gravel bike will take a 40mm tyre without complaining, and many will take up to 50mm. That width is the single biggest reason these bikes ride so well off tarmac. Wider tyres at lower pressures absorb the surface, stay planted on loose gravel, and save your wrists on long rides.
Beyond the tyres, the geometry is longer and slacker than a race road bike. The head tube is taller, the top tube is longer, and the bottom bracket sits a little lower. You sit more upright than on a road bike, which pays off over a four or five hour ride, but it costs you a little speed on smooth tarmac.
Cyclocross bikes look superficially similar but are different tools. They have tighter geometry, shorter wheelbases, higher bottom brackets for clearing obstacles, and rarely any provision for luggage. If the listing describes a bike as a cyclocross bike, assume it will be twitchier on long descents and will not take 45mm tyres.
Tyres and wheels, setting a gravel bike up for your riding
Tyre width is the dial that changes what a gravel bike can do, and most frames let you tune it. Around 35 to 40mm is fast and smooth for tarmac and light gravel. 40 to 45mm is the sweet spot for most UK riding, balancing comfort, grip and rolling speed. 45mm and wider gives real control on rough, technical ground. Because a frame with more clearance lets you change setup later, buying a bike with headroom to go wider is buying yourself options.