Guard Break is a move that, you guessed it, breaks through and hits a guarding opponent. You can find these moves in the Command List: they are the moves that have a pink GB icon in the help column.
You will most likely get this naturally playing through the various game modes, but if not simply grab a second controller, jump into Original Mode > VS Battle, and do it against an idle opponent. If you previously disabled Neutral Guard in the game settings, make sure to turn it back on first.
I used Nightmare's 4 move.