Hi there,
This issue can be easilt fixed..if I understand what your trying to do correctly.
U have a seperate DB table of cars, and the gu_id is like the ID that identifies a car?
If you add this as a TV, and create a container of "car" resources, you can link your modx resources to your custom table.
Then once a user hits:
http://yourdomain.com/cars/big-red-super-car, your page knows its id as its in a TV.
Now pull that record and display in a custom snippet, this help?
J