Switch screen resolutions:
Hold L + Start at the Capcom logo to switch to a low resolution mode. To switch back
to high resolution interlaced mode, hold Start + X at the Capcom logo. 

Fight as Gill:
Successfully complete the game. Then highlight Sean and press Up(2) at the
character selection screen. 

Alternate costumes:
Successfully complete arcade mode with any character, then save the game. Highlight
any fighter, then hold Start and press an attack button at the character selection
screen to change his or her costume color.