you can try by the post Various GP3 Editors and utilities...
[
www.grandprixgames.org]
there is the GP3 Edit version already instsalled in a folder. So you can run it just unzipping going in the folder and run the exe file.
There is also , Grand Prix 3 Master, which you can use for editing livries.
Anyway, mind you need to use the decrypted 1.13 gp3 executable version.