Not to insult you, but do you have the tech package in your GT that actually has the navigation?
haha no worries. Yeah, we have tech package. Bought the car back in Dec, have used the Nav several times.
Or, with the car turned off, open SD card's door and take the SD card out, pause a second, put it back in, close SD card door, and start the car. You'll have to wait a few moments while the system reboots, but this has worked for me when the TomTom gets fussy.