Drywall isn't considered a combustible in some applications, but behind a range in the kitchen, for instance, you would want cement board and metal studs if you don't have a backguard. Your area's buiding code enforcement office could probably help you out a lot here if you live inside city limits. I'd be more concerned about wooden studs behind the sheetrock that may experience heat transfer than the sheetrock itself. If this isn't a fireplace regularly used, consider an electric insert.