Cheats > Game > Cheat: 47667

Fight as Gouki/Akuma (versus mode)
7859

Choose versus mode from the title screen, then select the game speed with the Fierce Punch button. Highlight Ryu and press every button except the D-pad simultaneously. If you picked your character first, keep holding these buttons until the second player makes their choice. If the second player also wants to be Akuma he or she will also have to hold all six buttons.