I was finally able to get GPxPatch working in Linux. I had been using the original GP4.exe from the CD, and apparently going to the decrypted file fixed the issue of GPxPatch not recognizing GP4. It's now working.
As an additional question, with 'Power and Grip Team Dependent', will that also make me as the player have a 'random' performance variable from the performance.txt file, just like the AI players?