no you're not been silly - you definitely have to check it out with the car hire companies you are looking at. I'm not sure all allow it and those that do often add a charge. That said, I am sure a lot people flying in to Girona will want to cross into France so worth checking out with the various hire companies that service the airport. BTW Girona is a beautiful city if have time to visit it. Also being nosey - which campsite are you going to? We've done sites near Girona and Perpignan.