Verify that "Ocarina" or "Cheats" is enabled in your loader settings and that the .gct file matches your game's region (RMCE01 for US, RMCP01 for Europe).
The RMCE01 ID is most famously linked to (Custom Track Grand Prix). This massive mod adds over 200 custom tracks to the base game, ranging from original creations to remakes of tracks from Mario Kart 64 and Mario Kart 8 . rmce01
