Sweetleaf wrote:
metal side down on the seat and glass pannel up with nothing sitting on it aside from a blanket or two
^^^ This is how I prefer to transport my own personal gaming PCs, and generally recommend this method to others as well.
If it's in a back seat I'd put it behind the passenger seat and move the front seat back to pin the case from sliding forward. If it's in the front passenger seat, I'd move that seat forward to try to pin it against the dash / knee area, also to keep it from sliding forward.
Laying it flat like that takes a lot of stress off of any large tower coolers or large GPUs inside the case, and keeps them pressed together. Given that yours is a gaming PC, there's a good change it has a large GPU at the least, and a good chance of a larger tower cooler as well, unless it's liquid cooled.
Also reduces the chance of it tipping over if you lay it flat like that - always clear panel up / motherboard on the bottom.