Depends on how much riding you plan on doing. The actual mtb trials could be done on a gravel bike, just slower, but they are more enjoyable on a mtb. It’s not the most technical riding for a trial centre, don’t go thinking you are getting tweed Valley level of trials. But they are good for a quick blast around.
Other than that there are many miles of forest roads which are more suited to a gravel bike.
I suppose it all depends on what type of riding you want to do and how much riding you will do when you are there.
In my opinion it’s not a destination to go solely for the mtbing. For the size of the place there isn’t that much trials, There are certainly better places to go. But the mtb riding is ok if you are already going there for a holiday.