At the title screen, press start to get the mode select, and enter:

L-Shoulder, Z, R Shoulder, Up-C, Down-C, Left-C, Right-C, Start.

You should here a sound, not choose GP Race and a new difficulty will open up.

On the select course screen, a new set of random tracks called the X Cup are opened, and on the select machine scree, you now have access to every vehicle in the game.