To make it all work and do some tuning you'll also need GPxPatch which allows a number of track and performance files to be loaded at startup each session.
I also use GP3Physics editor, GP3Track Editor (Paul Hoad's one) and Cmagic which lets me put custom track data into the great tracks we now have. I use GP3Master to update the images used behind the menus with 2007 pics. Drafi5's site also has an update for the Belini Istanbul track. You need the Belini one first but the Drafi5 version does run on GP3 1.13 (that's the patched and decrypted noCD version you will be using.
If you use Windows XP SP2 you will need to do some track editing on a lot of circuits as there are items that SP2 doesn't get along with. If that happens do a search for 0xc5 and 0xc6 editing. (Don't worry, its not as bad as it sounds)
Apart from the item on Drafi5's site, everything else I mentioned is available in our old downloads section.
Have fun.
Nico bites yer tyres!