You don't edit the car lode.gp4 files.
You need to find the corresponding tex file for the car you want to edit, usually named as the team or car model and is a tex file.
You then have to use TexResizer to open the tex file and save the image as a tga file which then can edited using say PhotoShop.
LAMH DEARG EIRIN
GPGSL Alpha Motorsports, S13 Race Driver #8 S14 Race Driver #2, S15 Race Driver #2