If the ie4 is designed similar to the M4 & RM4, there may be a an internal dip switch on the midi board. Phantom power should active on pins 1 & 3 and 6 & 7 but if it's not powering the footswitch, check the internal dip switch (if there is one). Hope that helps.
Edit: I see, you want to power it from the GCX? You might want to power the switch from the ie4 and then midi from the ie4 to the GCX.