If your gravel bike has a 15x100mm front thru-axle, it almost certainly has a suspension fork. This is the same axle standard used on mountain bikes, and suspension fork manufacturers design their forks around it. The 15mm diameter (versus the 12mm that's standard on most gravel bikes) provides a stiffer, more secure front end, which matters when you're hitting rough terrain with a suspension fork.
The trade-off is wheel compatibility. Most gravel and road wheelsets are built for 12x100mm axles, so your options for swapping in a second wheelset are more limited. You'll need to look for wheels specifically designed with 15x100mm front axle compatibility, which often means mountain bike wheels.
For most riders, this isn't a big concern—you bought a bike with suspension for a reason, and the wheels that come with it will serve you well. Just keep the axle standard in mind if you're shopping for a backup wheelset or a replacement thru-axle, since 12x100mm and 15x100mm are not interchangeable.