For anyone having issues try the following:
1) Have you tried using the pre-patched, ready-to-use game available at GP4 Central?
2) Have you set "CPU = 0" and "Debug Verbosity = High" in GPxPatch?
3) Have you set GP4.exe to compatibility mode = "Windows XP SP3"?
4) Have you tried installing the game to your Home folder? E.g. C:\Users\MyName\GP4\
5) Have you tried inserting your game CD (or mounting a GP4 .iso) before loading the game?
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Intel NUC 8i3, 8GB RAM, MS Sidewinder Wheel