But the main thing to remember is that GMT (or UTC) is not the same as GPS time. When the satellites were launched, there was no way of predicting how time would change with all the little fixes that have been done to GMT to adjust for the changes of earth rotation speed. The difference is in the order of 2.5 seconds (I gather).
The FMS in my jet had a frig factor to convert the time signal to GMT, and actually calls it UTC (bl**dy frogs!)