American international Robbie , 27, scored six goals in 27 appearances across all competitions for Nottingham Forest last season, but he has not taken to the field for the Championship side this term.
He is expected to train with his new team on Friday and go straight into Gillingham's squad ahead of Saturday's match against Port Vale.
Findley, who was a starter for the USA national team during the 2010 FIFA World Cup, went to Nottingham Forest from Real Salt Lake after the 2010 MLS season.
He played in a couple of matches in the 2010-11 campaign before suffering an injury that sidelined him, but he came back and earned his place the following season under manager Steve Cotterill.
Nottingham Forest replaced Cotterill with Sean O'Driscoll ahead of this season, and Findley has not benefitted from the change, but he will be given the opportunity to earn some playing time in the fourth tier of English football to try and prove his worth.