Is it possible to control this gearmotor with the controller or do I need the more expensive / more powerful TReX Contorller?
The TReX Jr is a bit underpowered for that motor. If you want to be totally safe and have the ability to get the full power out of those motors, you should go with the larger TReX, but you could potentially make the TReX Jr work if you run the motor at its rated 7.2 V or below and avoid stalling the motors for prolonged periods of time (this is a generally good rule of thumb when it comes to the health of your motors). At 6 V, that motor has a stall current of 6 A, which is in the range where the TReX Jr’s motor drivers might start actively limiting the current. If you do use the TReX Jr, I suggest you use its built-in acceleration limiting feature to decrease the current draw when starting the motor from rest or rapidly switching direction (switching directly from full-speed forward to full-speed reverse can briefly draw twice the stall current, which could cause problems).
Thanks for the reply. I went for the more expensive option.