Wizz recently moved all routes from Girona to Barcelona El Prat.
And most flights from Rome Ciampino to Rome Fuimicino, so the precedence is there for Wizz to comfortably operate out of a "high cost" airport.
I believe that the Rome Govt insisted airlines to move at least some of their operations from Ciampino to Fiumicino because of things like noise levels, pollution, etc.... The airlines made a fuss, but eventually (apart from Ryanair) they did so in good grace.
Girona has a strong appeal to the holidaymaker who wants 2 weeks on a Spanish beach or to visit a theme park. It is not a good airport for those who want to travel to the city of Barcelona.