I had the same problem. The kind members of this community provided the solution and it works. This way you can use every 1.13 shape in gp3-2k as well.
Here's what you gotta do:
1. Load the 1.13 shape into car editor,
2. Go to "File -->Export CTF"and save the file as "custom.ctf"
3. Load any carshape that works with gp3-2k into car editor
4. Go to "File --> Import CTF" and import the "custom.ctf" to the shape.
5. Go to