Unlock all cars, tracks and modes:
Enter "ENCHILADA!" as your name.

Invincibility:
Enter "CODY" as a name.

Gate Rally Mode:
To unlock Gate Rally Mode, you need to accumulate one win in Smack Attack Mode.
An easier way to accumulate the one win you need is to plug in three additional
controllers and play on the space level. The three uncontrolled opponents will
always spawn on the UPper ring around the level. This leaves them wide open for
you to kill, but they're ignored by the CPU controlled players. You should be
able to rack UP enough points to win by killing these sitting ducks.