IMO, dog parks are not a good place for any dog, let alone a mastiff. The reason being, other pup owners do not keep close tabs on their pups. When a fight breaks out and a mastiff is involved, guess who gets blamed for the fight?
Go to human parks and walk on leash around people and other dogs. Watch Gigi's body language when people/other pups approach. I would avoid head on meetings with other pups and avoid people rushing her to pet her. If you see a pup and want Gigi to interact, have the 2 pups walk together first and when both pups are relaxed, then allow the meeting and sniffing to begin while keeping an eye on Gigi's and other dog's body language.
Take her for rides in the car, take her to petsmart, take her to festivals and other outside gatherings for socialization.