ok.
make a custom camera, remove the anchor and the target, go to your GPS's beginning, then go to your next point. click what time you want it to be on the block and put a key there. then do the same for the next point and so on,
but if you make your GPS so simple like using too little keys, lets say you have a big turn, and you put one key on one side and one on the other, the GPS will skip to the other side without turning.
but using too much will waste your time. also, putting them too apart makes the GPS slow, and putting them too close would make it go too fast. it is all about going in the middle.
EDIT: however you dont need a key for the first point

Here you can find the answer: http://nations.tm-exchange.com/main.aspx?action=threadshow&id=1203660&page=auto#auto

I made a tutorial which includes how to make a GPS...