In 1996, Gerstner joined the cover band scene in South Germany, where he met his former band mates from Freedom Call. He joined Freedom Call in 1998, with whom he recorded two albums and did three big tours in Europe before leaving in 2001.
After leaving Freedom Call, Gerstner co-founded a recording studio and worked as a producer and songwriter for other young artists until Helloween producer Charlie Bauerfeind contacted him and introduced him to guitarist Michael Weikath. In 2003, Gerstner joined Helloween. Aside from his work in Helloween, he also runs his own recording and mixing studio called G-Tracks.