I am not very fluent when it comes to using multiple GCX units, but the short answer to your primary question is yes, you can control both GCX units with a single Ground Control Pro. You'll find the complete instructions on Page 12 of the GCX Manual, but the short versions is:
Connect the Pedal In of the first GCX to the MIDI Out of the Ground Control Pro. This connection will also provide power to the Ground Control Pro.
Connect the MIDI Out of the to the first GCX to the Pedal In of the second GCX. Pleas take note of the warning on Page 14 stating the MIDI jumper cable should NOT have pins 1 and 3 wired.
Since you want to control three additional MIDI devices, you may need a MIDI thru box as recommended in the manual.
Hope this helps...
thank you very much