maybe carmodel (carshape) is for another version of gp3. there are 3 different version of gp3.
gp3 1.0 - just after installation . it's unsefull .
gp3 1.13 - when you install the official patch 1.13 , or download gp3 113 executable.
gp3 2000 - based default on 2000 season F1.
the car editor is settable for all both them. probably you are using a carshape created in another version instead yours.
try this.
open carshape you want to use. select File-> export ctf. save the file with a name.
reinstall gp3 import carshape from exe. select import ctf. take the file saved.
select exoprt to game.