From Alison Hine's site: http://eaglewoman.simracing.dk/gpl/index.htm
5.3 Why can't I race against the AI while driving a Trainer?
Actually, you can. You have to change one small item in your player.ini file. I recommend you create a new player just for racing in each of the Trainers. For example, I have three drivers: Alison Hine (for the GP car),
anbei die leider englsche Anleitung zur Einstellung der KI
Ali-F2 Hine (for the Advanced Trainer), and Ali-F3 (for the Basic Trainer).
Once you've created the extra player or two, open the GPL players folder (by default, C:\SIERRA\gpl\players) and then open the folder for one of the training players. For example, I would open C:\SIERRA\gpl\players\Hine__Ali-F2. Now, using a text editor, edit the file called player.ini.
Look for a parameter called driverRating. It's in the [ Personal Information ] stanza. Change the value of driverRating to 2 for the Advanced Trainer, or 1 for the Basic Trainer.
Now, when you race as one of these new drivers, the AI will go slower.