Gills chairman Paul Scally told the club's official website, "Darren is a fantastic signing for this football club, and the addition of another quality forward to the team speaks volumes for the type of attacking play our manager wants to play next season.
"We are an ambitious club and the signing of quality players like Darren can only help us in our quest for top-flight football."
The arrival further strengthens Gillingham's attacking options - and following a poor season in front of goal last season, a new-look strikeforce of Byfield, Agyemang and Roberts bodes well for Gillingham's prospects this season.