Gersbach returns to Ligue 2 - the French second division -having previously played at now-Ligue 1 Lens on loan from Rosenborg in 2018.
The six-times capped Socceroos star spoke of his excitement at returning to France.
“I am very happy to come back to France and Grenoble," Gersbach told the Grenoble website. "I have already played for six months in the country, at Lens.
"I had a very good experience there and I have only positive things to say about the Ligue 2 championship.
"I am ready to give everything to be on the field and show what I can do."