The French striker scored twice at Stade Marcel Picot to help the former French champions register a 3-0 win against the relegation-threatened side.
"It was a difficult match," Garde told reporters. "We met a Nancy team that were on a very, very good run. They're practically one of the best teams of 2013. Gomis was really good when he came on.
"I have to admit that I only made the change because Lisandro [Lopez] had a calf injury. Gomis took very little time to get into the match. I think he contributed greatly to the win.
"After Nice's win, we had to win. But there's still a team behind us - whether it's Nice, Lille or Saint-Etienne, who remain threatening for us."
The win saw Lyon remain third in the league table with 63 points from 35 matches.