The former Watford, Stoke City and Burnley frontman, 28, had been playing in Spain for Elche, but was allowed to leave the club at the end of last season.
He had been training with the Lions and the manager Kenny Jackett is delighted to have finally completed a deal.
He told the club's official website: "Gifton is a very talented player who, at 28, still has plenty to offer.
"He's been abroad in recent years, but wants to get back into English football and I'm hopeful this is the right club for him.
"He's very hungry and has enough ability, determination and personality to do very well here."