When your PC crashes and takes out a crowd of people...
jk, but electrically the car is probably fine. Most model cars have clear coat over the paint so you should be covered there(edit: lol the pun). I would just secure the car wherever you place it, since PC's tend to be moved and bumped. You can place the car on the GPU, although I would avoid it due to thermal reasons(meh) and to prevent even more GPU sag(and depending on the weight of the car, it could strain the card/slot).