MIDI Channel
The GCX responds to MIDI Control Change messages sent on 
channel 16 only. 

Control Change
A data value of 0 turns the loop off, a data value of 127 turns the 
loop on. Controller numbers are as follows:

Audio Loop or Latched Switch
 
Loop #1 = controller #80 
Loop #2 = controller #81 
Loop #3 = controller #82 
Loop #4 = controller #83 
Loop #5 = controller #84 
Loop #6 = controller #85 
Loop #7 = controller #86 
Loop #8 = controller #87 

Momentary Switch
Loop #1 = controller #104
Loop #2 = controller #105
Loop #3 = controller #106
Loop #4 = controller #107
Loop #5 = controller #108
Loop #6 = controller #109
Loop #7 = controller #110
Loop #8 = controller #111