To set up a Boss ES-8 with a Mini Amp Gizmo or Amp Gizmo, you need to set up the ES-8 to send MIDI Program Change messages to the Gizmo. Follow these steps on this web page to program the ES-8:
Follow that process, setting channel (CH) to 1 for each preset. And, assign each preset with a different PC number. Preset 1 sends PC 1, Preset 2 sends PC 2, etc. You don’t need to set anything for CC#. Do this for each preset you’re using on the ES-8.
After you do that, turn on some switches on the Amp Gizmo and select a preset on the ES-8. The switches on the Mini Amp Gizmo should automatically turn off if things are set up correctly.
Assuming that it’s working, you can now program the Mini Amp Gizmo - select a preset on the ES-8, then use the front panel of the Mini Amp Gizmo to select the desired amp channel (and other features, if applicable). Hold the Write button, and the lights should flash after 3 seconds. Now, when you select that preset again, the Mini Amp Gizmo should automatically turn on the desired channel. You can do this for each preset on the ES-8.