Instructions: 

1) Create 2 new drivers in GPL, naming them as normal, but add F2 & F3 to the end of the FIRST names. For example, I named the drivers SamuliF2 Takala & SamuliF3 Takala 

2) Exit GPL, and open the file gpl\players\your_nameF2\player.ini for editing (by doubleclicking it) 

3) Find the row driverRating = 3, and change the number to 2 (1 = F3, 2 = F2, 3 = F1) 

4) Repeat steps 2 and 3 for the file gpl\players\your_nameF3\player.ini, but this time change the number to 1 

Now you're set to race with the Trainers and Advanced Trainers, just select the appropriate driver and join the race!