The first thing to check: Does the Amp Gizmo change channels on the amp when you press buttons 1-3? That needs to work before setting up the GT. If it doesn't, you may have a defective cable or defective Amp Gizmo. If it kinda works, but does some weird things, the Amp Gizmo needs to be set up for the cable. The cable should have come with an instruction sheet. I have attached it just in case.
Once the amp is responding to the Amp Gizmo, you can get to the next steps:
- The Amp Gizmo's internal MIDI channel has to be set to match the MIDI channel number that you set in the GT's Devices tab. If you set it to channel 2 in the GT's settings, set the Amp Gizmo itself to channel 2. I have attached the Amp Gizmo manual in case you need that - the Setup Mode section shows how to set the MIDI channel
Then, you can set up the buttons. Your first post is closer to being correct, but you do need to set all of them to the same group number. The second one post is not correct, you should delete that PC action from the button if it's still there. It can cause problems.