Gradius: The Interstellar Assault (or Nemesis II: Return of the Hero) is the second Gradius game for the Game Boy. It may well be the only Gradius game that doesn’t have Moai heads. A colour version of the game can be found on Konami GB Collection Volume 3 for Japan and Volume 4 for Europe.
Nemesis is the first portable Gradius game, and the only one to use the original European name of the series worldwide. The game is a weird combination of elements from both Gradius and Gradius II: Gofer no Yabou. Like many of Konami’s early Game Boy efforts, a colour version was released as part of the Konami GB Collection series.
Another Gradius side story.
Gradius Galaxies takes place between Gradius III and Gradius Gaiden, with a previously forgotten Bacterion weapon being rediscovered and the Vic Viper being sent to destroy it. Pretty standard Gradius fare, though that’s not exactly a bad thing.