Compatibility summary
You will need a travel adapter in Guernsey; plug types used there differ from France.
Plug comparison
France plug types: C E
Guernsey plug types: G
There is no common plug type between France (C E) and Guernsey (G). You need a travel adapter.
Voltage comparison
France: 230V, 50Hz
Guernsey: 230V, 50Hz
Voltage difference is within a range many devices tolerate; check your appliance label.