A look at gas log fireplaces themselves gives good hints by the manufacturer. They do not make their fireplaces of paper-covered gypsum with an overcoat of combustible paint. Metal sheet with baked enamel paint is the norm. Most appliances require at least an inch of spacing from combustible materials, which includes drywall walls. I would feel safer with a metal cover. A separating sheet with a wire mesh could give a base for gypsum or cement stucco, but this could crack unlike the metal itself.