I have now looked at the schematics for all the aircraft that I have access to. These range from a MSN in the 400s to some in the 2000s and, without exception, when fitted with GPS then the clock uses GPS system time as it's input.
Now I'm not saying that this will be the case for all 320s fitted with GPS, that's what options are for, but Airbus are less free with their options and differences than that other manufacturer.
It seems to me that it is logical to use the best time source available and if you have GPS then that's it.