[pirenzo]

The problem with making mods for GPL is the physics are not easily edited, they weren't suppoed to be, quite the opposite in fact. With such a hard game, you'd want to make it difficult to hack to deter cheaters.

What the general process is now for making mods for GPL is that someone has to produce the carset, and then the one or two people in the know (literally only 1 or 2, I think its Richard Cooke and Phil Flack (Guru)) will make the physics for it.
Currently proposed or in progress mods are:

F1 '69 (nearest to completion)
Sportscar '66/67 (GT40, Chaparall 2F, Ferrari 330 P4 built, and in testing)
Sportscar '70 (Porsche 917K soon to be released)
Caterhams (original test mule for altering physics I think, someone started making the car)
F1 '66 (carset already in existance, but no work on physics as yet because too similar to '67 to be worth it)
Tasman '68 (first attempt at physics completed, no cars)
USAC '67 (no cars or physics AFAIK)