On the A320, it's an option. Our fleet, some do - some don't depending when they were orderd and which clock they have. They all display seconds rather than decimals.
Also, on the newer aircraft with Pegasus, the display does actually show UTC (GMT if you're British!) wheras from what I remember of the Boeing FMCs which dispalyed GPS time. There is a difference! This is because when the first GPS satellite was launched, it was set to UTC and since then, every satellite has had to follow the set time. However, UTC has undergone several corrections since to allow for variations in the Earth's rotation rate. I don't know what the difference is, but is now a few seconds. The Airbus FMGC software has a correction factor.