The bad news is that there is no Vehicle Speed Sensor on the GT's.
But the good news is that the PCM uses the transmission OSS (Output Shaft Speed) sensor and parameters programmed in the tune to create a VSS signal on pin 68 of the PCM.
More good news is that it can be tapped at the radio, as shown in the diagram below:
It is fed to the radio for the vehicle speed controlled volume setting.
I find it curious that the GPS should require a VSS signal, as a GPS is entirely capable of calculating speed all by itself--and far more accurately than the vehicle's system. With 4 or more channels locked in a GPS is accurate to 0.1 mph--maybe it's so the GPS can display something if it can only lock onto 1 or 2 satellites?