Print, clean up, file a flat on the motor's shaft if necessary. Fit the hose clamp loosely over the collet. Put the thickest quadrant of the collet over the flat on the shaft, and push the gear onto the shaft - the hole tapers. When it's at the right level, just do up the hose clamp. The gear will be held in place quite securely.

In PLA the hose clamp may crush the collar, or may simply foul on the surroundings. In that case, just epoxy it onto the drive shaft.