It isn't.

Gravel bike is just a new marketing term.

Gravel bikes are more sporty. They come with drop bars and geometry to complement that riding position, and will weigh much less than an upright hybrid.

You’ll also find gears to accommodate more speeds and inclines, typically a single chainring with an 11 speed lightweight derailleur vs 3 to 9 speed heavyweight (but clean and easy maintenance) hub gears.