Gillett, who has signed a two-year-deal at the City Ground, admitted that reuniting with former manager Sean O'Driscoll played a major part when he made his decision.
Gillett told the club's official website: "The club speaks for itself with a great stadium, great fans and great history.
"But the manager played a massively important part for me as well. I know how he works and I feel he can get the best out of me."
Gillett is Nottingham Forest's fifth summer signing after Adlene Guedioura, Danny Collins, Greg Halford and Dan Harding joined the club.