Isn’t it just a gx16 3 pin? 
No.
GX12 3pin? 
The third pin is the NTC, The charger needs to see the resistance within the tolerable range before it will charge. Pretty sure the pinout is labled on the charger, and the GTR battert PCB is labled also for it.
I’ve just spent too long trying to repair a friends GTR ESC. What a waste of time that was.
The GTR charger just needs to read a 10k thermistor between the purple and black on the charge port and it will charge.
When I had a gtr I put a vesc and new battery and used the stock gtr charger no problemo
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I did just that, and put some of those little three pin amass connectors in the cord - like the ones on the GT motors, so it can be used with different output plugs.
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