Gartner was drafted in the 1st round, 4th overall, by the Washington Capitals in the 1979 NHL Entry Draft. He started his professional career in the WHA as an underager with the Cincinnati Stingers in 1978–79. He played on a line with Mark Messier and had a successful campaign, finishing second to Wayne Gretzky for Rookie-of-the-Year honors. With the WHA folding at the end of the season, he played the next season with the Capitals, and would play for them for the better part of ten seasons. He recorded an assist in his NHL debut on October 11, 1979, versus the Buffalo Sabres, and he wore number eleven for the Caps.