Gomis netted his sixth of the season to give new American owner Frank McCourt some hope ahead of the first match under his tenure, due on Sunday next when champions Paris Saint-Germain will provide a daunting test.
Despite their underwhelming start to this campaign, it was at least a third home win this season for Marseille -- as many as in the whole of the last campaign where they finished 13th.
But the game itself was forgettable in a Velodrome that was well under half full.
The winning goal came on 13 minutes as Gomis was quickest to the rebound after Metz goalkeeper Thomas Didillon had pushed out his header from a corner.
With the OM under his belt, McCourt becomes the latest American tycoon to take control of a European football club after Manchester United, Arsenal, Liverpool and Roma all have US owners.
"I don't know anything about the new owner, but I hope that he'll be at the top and will give us a lot of pleasure" Michel Tonini, the head Yankee Nord, a Marseille's fan club, told RFI."Nothing has been good for these last year, so we hope that everything will be a new adventure."
McCourt is the former owner of the Los Angeles Dodgers but his time there was deeply controversial.
McCourt's turbulent eight-year Dodgers' reign was marked by friction with Major League Baseball, who eventually seized control with commissioner Bud Selig citing "deep concerns regarding the finances and operations" of the team.
With McCourt struggling to meet the Dodgers' payroll demands, the team filed for bankruptcy protection. The beginning of the end came in November 2011, when McCourt agreed to sell the Dodgers in a process supervised by a federal bankruptcy court judge.
He remains the owner of the successful Los Angeles Marathon and in 2014 took a 50 percent stake in the Global Champions Tour show jumping series.