Gersbach, who joined in January, played the full 90 minutes as a second-half brace from captain Mike Jensen and a goal from Christian Gytknaer helped Rosenborg secure their third Norwegian title in a row.
The former Sydney FC defender, played 17 times in the league this season, picking up two assists in the process.
The win for the Trondheim club, who have five matches left to play, puts them on 62 points after 25 games, a whopping 21 points ahead of second-placed Brann Bergen, who have a game in hand.
"It feels fantastic, it's hard to describe," a jubilant Jensen said after the final whistle. "Everyone in the team has been focused on this and to finally stand here as champions is an incredible feeling."