The 26 year old midfielder is linking-up for the second time with Sean O'Driscoll by joining The Reds as a free agent.
Forest boss O'Driscoll took Gillett to Doncaster from Southampton two years ago and his career blossomed at The Keepmoat Stadium.
Forest Chairman Omar Al Hasawi said: "Simon is a player that Sean knows well from his Doncaster days and we are delighted to welcome him to the Club."
Gillett launched his career with Southampton and made more than 30 appearances for the south coast club.
He spells on loan with Walsall, Blackpool (twice), Bournemouth and Doncaster and O'Driscoll swooped to sign him permanently in the summer of 2010.
Gillett was an ever-present for Doncaster in the Championship last season and scooped various Player of the Year awards.
His contract expired in the summer but Doncaster were unable to offer him a new deal following their relegation to League One.