It's a fairly common problem with those running the GTB as it's reasonably heavy.
Try moving the electronics inward towards the centre-line of the chassis. It will definately help.
If there's no more space, then it's time to add lead weights to the battery side. I suspect there isn't enough space to mount the GTB on the battery side anyway.
Lead weights are available as lead weight sheet from Tamiya, or blocks from various manufacturers. Another cheap source (cheapest by far!!!), is your local tire store. They'll have adhesive lead weights for wheel balancing on full sized cars.
Usually come in a strip with small sections for 5g and 10g that you can snap off.
Mount small weights outboard to balance the car with the minimum amount of weight required.
Running MOD - the extra weight is a small penalty, for a much better balanced car. Less so for the classes where power is at a premium...