I use my GCX's with the old style Ground Control pedals. I really like those pedals and they work perfectly with the GCX. The GCX will also power the pedal using a standard 5-pin midi cable (don't need a 7-pin).
The older/used Ground Control pedals are a great deal for the money.
I tried using my GCX with an ART X15 Ultrafoot (my favorite pedal). I quickly deduced that the low confusion factor was worth the $399 for a compatible pedal board (GC Pro).