Hi Hosie,
The feedback from the site indicates that you are not uploading a native Navi GT file.
The best files to upload are ones with filename extensions .SBN or .SBP.
The file you are uploading is a .GPX file.
If your device is really set up as per the instructions at
http://mtbest.net/setting-up-navi-GT31.htm then you should be OK. You may have just missed a step, so go back and re-check.
Can I ask how you are transferring your file to the computer?
The best way is usually to dump it onto an SD card in the device and then remove the card and read the file on your computer via a card reader.
If you are trying to transfer data via the USB cable, this isn't so reliable. Most people I know use the SD card.
Persist with it. Once it is set up right, everything will run smoothly. I remember getting our first NAVI (I think it was a Christmas present, so at about this time of year.) Nobody seemed to know how to get them set up right then. These days the info is pretty detailed.
Dylan.