The 27-year-old attacker grabbed his sides fourth goal of the match after having fallen 2-1 behind to the Ligue 1 side during the first half.
Osimhen spent a single season at Lille, scoring 18 goals while providing six assists in 38 appearances before making a switch to Serie A side Napoli in 2020.
"I love this stadium [Stade Pierre Muaroy]. There is an incredible fan base," said the striker to the media after the match.
"We played a difficult match. I am very happy for our team, we won. I wish Lille success for the next matches. Lille players are very tough players.
"They also took the lead. We managed to win the match with the team. I'm happy we got the three points."
The result marked Gala's first win of the 2026/27 UEFA Champions League campaign, moving them up to 23rd on the table, level on points with Osimhen's former club Napoli who occupy 22nd position.
He has scored seven goals and provided three assists for Galatasaray in eight appearances this season.
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