Is it harmful for my son to use Premium gasoline in his 1997 GMC Sierra pickup?

No, using premium gasoline in a 1997 GMC Sierra pickup is not harmful. While it's not necessary (the truck is designed for regular gasoline), it won't damage the engine. Premium gasoline simply has a higher octane rating, meaning it's less likely to pre-ignite (cause knocking) under high compression. Since your son's truck likely has a lower compression ratio than modern engines, the benefits of premium fuel are minimal and he won't see any performance gains. He'll just be spending more money. It's perfectly safe, though.