32-year-old Butters has signed a two-year contract with the Seagulls after failing to fit into any of Gills' player-manager Andy Hessenthaler's plans so far this season.
Butters links up with Gills old boys Richard Carpenter and Paul Watson at the Withdean Stadium.
Hessenthaler told the Gills official website: "We are sorry to see Guy leave us, because he has been a great servant to the club and a great character to have around. We wish him all the best except, of course, when he's playing against Gillingham."