Alright so heres a game i've been playing for 2-3 years. its still in alpha, so don't expect much. Its a Physics engine based on rigs and rods (sticks and stones) to connect the parts together. the result is a fantastic crash physics, realistic trailer loading characteristics, and very specific controls. The controls will take a while to figure out, but after that, your set .
The game has a number of vehicles. cars, trucks, planes, boats, helicopters, etc. Its an open source game so its free and there are always new mods to be added onto the game.
Heres the link to their site
http://repository.rigsofrods.com/

Unfortunatly, nobody has really made any GM cars except the 1st gen *-10. I hope you enjoy this game as much as i do