New Saddle For Gravel Bike
Leather is fine to ride in the wet.  However because its an organic product you have to treat it periodically with a preservative.  Also you don't want to just leave it in the rain all the time, that will accelerate its failure.   Having decent mudguards/fenders will help reduce water up from below too.

Covering the leather with a plastic bag helps reduce absorption of water, but you can't just wrap it and forget it - that way leads to mold and tears.

The saddle needs a chance to dry out, so store the bike inside in the warm.  If that's not possible then bring the saddle inside after a wet ride.

Brooks recommend their Proofide mix which is about 15 euros for 40 grams so 5% of the cost of the saddle.  I've treated other leather goods (belt, gloves, leatherman pouch) with various mixes of beeswax for suppleness and waterproofing.