EDIT: problem solved, big thanks to @Cyan
Put your GCT files onto the same device as you have the game ISO, and specify the path in USBLoaderGX Custom Path
Example:
GCMainPath: usb1:/games/
GCTPath: usb1:/codes/GAMEID.gct
And DIOS MIOS find the codes. ;)
If you want play a Wii game with codes...