GPS tracking might not work as you wish with TTN.
The fair access policy limits you to 30 seconds of air time per day, so if the thing you are tracking is at a long distance and the node goes to long distance mode (Spreading Factor 12) your limited to around 23 packets or transmissions per day …
If the thing you are tracking is in close, then you can use shorter range and less air time packets (Spreading Factor 7) and can have around 500 packets per day.