Hi, running costs are tough to pin down but certainly fuel efficient the GTO isn’t! Less than 20mpg with mixed driving is all you’ll get; mid 20’s at best on purely motorway cruising.
The rest of the cost (aside from insurance and the like) is maintenance (and tuning!). They’re often repairable with second hand parts for not much money, but of course there are things that are always best bought new. These are always expensive!
I’m not sure if you’ll be able to view these links but there’s a buyer’s guide here and a frequently asked questions thread here.