Gill, 38, was told he was surplus to requirements at Whaddon Road when new manager Martin Allen took over last month and the former Birmingham City and Northampton Town full-back has now joined the Blue Square Premier club.
He has joined them on an initial loan basis after three-and-a-half seasons with the Robins.
"It's a great opportunity for me to start off my coaching career and the other thing that appealed to me was the chance to keep playing as well," he told the club's official website.
"I think Forest Green have the potential to be a Football League club and I would like to play my part in helping them to achieve that."