Ok, I downloaded DVDdecrypter and here's the result I got.
http://www.geocities.com/rengokutm/gtconceptDVD9.jpg
Copy the link above and paste it in the address bar to see it.
Looks like this is a DVD9 disc after all - check the red arrow on the picture - 2 layers!
Though DVDdecrypter says the disc size is around 8 GB (check the red arrow with the ? in the pic). Windows only detects the disc to be 3.96 GB - so is this a windows bug or is it just my DVD drive.