I just recently found a source of 14 guage military grade aerospace wire, and its diameter is actually smaller then that of normal 16 guage. So the wires actually fitting in the GB shouldn't be a problem now.
As for the use of it in the G&G m14. Did you do anything special to fit the wired into the wire terminals?
You could also look at teflon coated wire, its thinner. However, unless you absolutely need 12-14# wire, it really isn't worth the trouble.
The way the Cheetah 2N Mosfet works, it has 6 wires coming from the unit.
2 to the trigger contacts (these don't need to be ver large at all, I asked Terry/Gandolf, and 24# wire is plenty)
2 to the motor
2 to the battery
The G&G M14 has the motor behind the gearbox, so all wires have to run past the motor to get to the battery compartment. Stock wiring is 18# wire running from the contacts to the motor and battery. The tight fit for this AEG is by the gearbox itself, and since the mosfet will be in the battery compartment, the only wires that will have any tight fitting will be the trigger contact wires. Since I will be using 24# wire for that, its no problem at all.
I'm not sure what you mean by terminals, but I also rewired to Deans connectors.