It's more likely a problem regarding the decrypted exe.
Did you install the game this way :
1- install the game from your CD
2- apply 1.13 patch (you have to execute it)
3- swap the original exe with the decrypted one.
Be sure to use the good decrypted exe : GP3, not GP3-2000.
Else, how many GP3 folders are you using ? In my case I got 4 folders for some custom championships. Your editors must point to the original one if you want them to work.