Carbon is not as fragile as many think. The tubes on such bikes will be built to handle typical riding for that bike - for gravel bikes this is probably not smashing the down tube onto a rock, but it will handle rocks flicking up and hitting the down tube without suffering structural damage, A typical Carbon MTB should be able to handle a reasonable knock to the bottom of the down tube with no damage, even though this is rare. Anything that risks smashing the down tube is unridable, and the chain ring usually gets in the way first if the timing of a jump bunnyhop etc is that far out.
Adding protection robust enough to make a difference comes with the cost of extra weight and ugly. You would probably get a lighter total setup with an alloy frame.
There are clear wraps available, that stick onto frames, for those who like to prevent chips and scratches and keep their bikes looking as close to showroom new as possible. These do provide good cosmetic protection.