The Gills have agreed a compensation fee for the 21-year-old, who turned down a new deal at Griffin Park.
The former QPR youngster spent a month on loan with Gillingham last season, scoring one goal in four starts, and boss Martin Allen is confident he can bring the best out of a player he calls 'The German'.
"He was deemed not good enough for Brentford and it's my job to help him, support him and guide him in the right direction, to help himself and of course score goals for Gillingham Football Club," Allen told the Gills' website.
"The German proved, in that pivotal game against Plymouth, where he scored one goal and made one, that he can do it.
"He doesn't know it yet but he will be pushed and pressured into the toughest pre-season, both mentally and physically, to maximise his undoubted ability."