The victory sealed a triumphant return to the club for veteran coach Fatih Terim, who took the reins for the fourth time last December, months after leaving his job as Turkey coach amid controversy over a street brawl.
Gomis, who has spearheaded Galatasaray's attack, took his season's goal tally to 29 with his 66th-minute spot-kick.
Fans of Galatasaray drove through the streets of Istanbul and Ankara, honking car horns and waving flags in the clubs red-and-yellow colours, while others set off fireworks and flares.
Galatasaray's nearest challengers, Fenerbahce and Basaksehir, both three points adrift, each won their final matches 3-2.
Istanbul's other major club, Besiktas, won the title in each of the last two seasons.
Galatasaray, who also won the UEFA Cup in 2000, last won the domestic title in 2015. They are now two titles ahead of Fenerbahce, who won it most recently in 2014.
