There is a Factory Reset option, and if used, the baud rate will need to be changed back to 56k.

Unplug the PI power first.
1. push #4 - 1 time - Main Menu
2. push #3 - 3 times - Init Setup
3. push #4 - 1 time - Init Setup Menu
4. push #3 - 3 times - Factory Reset
5. push #4 - 1 time - if No, push #4 again - Yes
6. push #1 - 1 time - Main Menu
7. push #3 - 4 times - Others
8. push #4 - 1 time - Others Menu
9. push #3 - 3 times - Comport Baud Rate
10.push #4 - as many times until 57600 BPS
11.push #1 - 2 times - exit Main Menu

Try it and then plug the GMC-320 back into the PI and then plug power on.
Harlan