RE: Good Service..........Bad Service......
Some gear boxes are slightly adjustable as the slots for the screws will be elongated. If the gears are too tight they will bind and cause higher amperage draw and possible damage to the motor, speed control or the gears. Too loose and the gears may strip out from not having enough surface contact. Some people will use the thin peice of paper inbetween the gears. I install the motor and slowly adjust the motor while rotating the gears to find the best lash with the least bind.
I am not familiar with the motor and gear box combination you purchased but take a look at it and see if the mounting holes for the motor are elongated or oval shaped.