Gartner was really a prototypical winger for that era in hockey before power forwards became "a thing". He was a tremendous skater, had a hard, accurate slapper, and could make plays at top-speed. His speed coming down the wing backed-off defensemen and allowed him the space and time to unleash his shot. I'd think that coaching and more disciplined defensive play and gap control nowadays (not to mention better goaltending) would negate some of that, but I have no doubt Gartner would carve out a nice career for himself in today's era too with his elite speed and shot. Kessel is a really good comp imo, but Gartner was faster. No physical game to speak of, and his reputation as an underachiever in the playoffs certainly weighs on his legacy, but you could pencil the guy in for 35+ goals for nearly a decade and a half, which is quite impressive.
Also worth noting that Keenan matched-up the Messier, Gartner, Anderson line against the Soviets' KLM line in the 1987 Canada Cup finals. Gartner wasn't someone you'd consider a defensive stalwart, but his speed allowed him to get back fast on the backcheck and he rarely got caught too deep or out of position.