"Magic data" is originally in the gp3.exe.
"Cmagic" allows you to extract this data in the form of a text file (which you can edit with a text editor) and then enter it into a track dat file.
Using the "GPxTrack" extension of "GPxPatch" makes the game read this data from the track dat files rather than from the gp3.exe file.
You will need to download and install GPxPatch and either Cmagic or Cmagic-2000 (depending on which game you are playing, GP3 or GP3-2000) in order to be able to do this.