To unlock Golden Mask in story mode, you'll have to beat the Seven Devil Choujin story just like in the manga. This is accomplished by beating certain oppenents with specific characters. After beating Stecase King and Blackhole with Kinnikuman beat Springman with Wolfman/Rikishiman, The Montain with Terryman, Atlantis with Robin Mask, Mr. Kamen with Brocken Jr. and Buffaloman with Warsman. Note that it doesn't have to be in order but you must make sure not to be defeated or the story will go on differently and you'll end up with a Game Over. After you meet these requirements the story will go on much longer and after finishing it you will unlock the Golden Mask Story.