I can make an educated guess based on page 10 and 11 of the manual here:
http://www.ripmax.com/Instructions/p-cb6l.pdf
I believe the standard setup with steering on right stick and throttle+gear on left stick would be:
channel 1 (right stick side to side): steering
channel 3 (left stick up/down): throttle
channel 4 (left stick side to side): gear
If you switch the gear servo over to the channel 5 switch you'll only get 2 gear positions. Channel 6 is the rotating knob.