Compatibility summary
You will need a travel adapter in Guam; plug types used there differ from South Korea. Voltage differs by more than 20% — a voltage converter may be needed for some appliances.
Plug comparison
South Korea plug types: C F
Guam plug types: A B
There is no common plug type between South Korea (C F) and Guam (A B). You need a travel adapter.
Voltage comparison
South Korea: 220V, 60Hz
Guam: 110V, 60Hz
Voltage converter recommended (South Korea 220V vs Guam 110V).