Tatar scored on the power play 12:41 into the second period then added an empty-net goal for some insurance in the final minute of play.
The 22-year-old won the Jack A. Butterfield trophy as the most valuable player in the AHL playoffs after totalling 21 points — including a league-best 16 goals — in 24 post-season games.
Defenceman Brennan Evans had the winner for Grand Rapids, the AHL affiliate of the Detroit Red Wings. Mitch Callahan had a goal and an assist for the Griffins. Joakim Andersson also had an empty-net goal.
Petr Mrazek made 17 saves for the win.
It is the first league title in the 17-year history of the Griffins franchise.
Richard Panik gave Syracuse a 1-0 lead near the end of the first period. Andrej Sustr also scored for the farm team of the Tampa Bay Lightning. Goaltender Cedrick Desjardins stopped 13 of 16 shots for the Crunch.
Grand Rapids went 2 for 4 on the power play, while Syracuse couldn't score on six man advantages.