Incase youre still wondering.........
The power supply for the Ground Control PRO is:
9volts AC
500ma
and the plug is "N" style which is 5.5mmx2.5mm which is slightly larger inner diameter than most standard barrel plugs which are usually 2.1mm
The Ground Control PRO accepts 5 pin only, using the outer 2 pins for phantom power. If you wanted to use the triaxis to phantom power the GCP, first double check that the voltages will be the same (make sure its only 9V and only AC power, not DC). If the voltages line up, then you can make an adapter box if you're handy with a soldering gun. Just get a 7 pin MIDI Jack, an N style barrel plug, and a 5 pin midi jack. Wire the corresponding midi signal only pins from the 7 pin to the 5 pin, then use the corresponding phantom power pins from the 7 pin jack and wire it to a barrel power wire or a barrel jack. Then plug it into the ground control.
Or you can get creative on the opposite side. At the rack, make an adapter to turn a standard 5 pin cable into a 5pin+phantom power. (3active pins for the midi signal, then get a DC wall wart, and splice it into the remaining 2 pins for phantom power).
I'm currently building a rack interface and breakout box for my rig so i can just send the 9VAC through an XLR connection from my rack, to a breakout box on my pedalboard which will be wired up to a N style barrel for my ground control, so i dont have to worry about the phantom power at all.