Facey, 27, was, after Will Hoskins, the Millers' top scorer last season. Despite runs of poor form and several listless performances early in the season, Facey showed up well in the final stages of the ultimately fruitless battle to avoid the drop down to League Two.
Facey's move south sees him link up with Ronnie Jepson, who was a team-mate of his when he started out at Huddersfield Town. Gillingham become Facey's tenth club in eleven seasons as a professional, in which he has played 250 games and scored 49 goals.