I wouldn't say Gaban *forced* to Imu use a new form but Gaban last moves made him take this battle more seriously, and it's consistent with Imu praising Gaban.
Since Oda is fan of Dragon Ball, there this part with Gohan vs Third From Frieza, where an angered Gohan used a massive blast that shocked Frieza by its strenght and had him go final form as a result even if Frieza ended up deflecting he blast without taking any damage (except maybe some fatigue) at the end.