Pagzy is right, CC Link can be used to control any IA button. I'm going to repeat what was said above, but with a little more detail...
- Create a new device in the Devices tab to represent your computer or MIDI controller that's going to be sending to your GT
- Make sure the MIDI Port is set correctly, and set MIDI Channel to the channel the computer will be sending on
- Set up an IA button, make sure that CC Link is turned on
- The first action in the list needs to be a CC Toggle action. Set it to the device you created in the first step, and set the CC number to be the one you want to control the button
Now, when the GT receives that CC from that device, it will set the button to be on (if the value is 64 or higher) or off (if the value is 63 or lower). Any actions after the first one will be executed as if the button was pressed.