Gradel, who has been on loan at the Yorkshire club since October, began his career as a trainee at Leicester but has found first team opportunities hard to come by at the Walkers Stadium.
Gradel has impressed for Leeds during his loan spell, scoring three times in 15 appearances and becoming an instant fans favourite due to his enthusiasm.
Boss Simon Grayson told the club website: "He is a joy to work with. He is a good lad, enthusiastic and always has a smile on his face. He's a good player and has brought a lot to this club.
"The crowd have taken to him from day one and he is probably the only player I've signed that after seeing him for just 10 minutes everyone wants me to sign him up full-time."