I do not know if there is a proper way to do this with files. But on the UI, make sure that the GT3 (the mod) entry is selected on the opponents filter. This is selected by default anyway when you use a GT3 car (from that mod). Put in many opponents, 2-3x more than you will actually use. Then before racing, boot the ones you do not want, then restart the weekend to remove the grid gaps for the booted cars (if going straight to racing). I can't remember if the add AI works on the new UI (did not work on the old), so that's also another way. If you boot cars on the new UI, you have to restart race (or weekend?) to show the change.