Cameo

Special guest appearances of Gradius stuff in other Konami games.

PlayStation 2   Airforce Delta: Blue Wing Knights
Xbox   Airforce Delta II
Game Boy Advance   Akumajou Dracula: Circle of the Moon
PlayStation Saturn and Others   Akumajou Dracula X - Gekka no Yasoukyoku -
Mega Drive   Animaniacs
PlayStation 2   ANUBIS Zone of the Enders
Super Famicom   Axelay
Arcade   Battle Tryst
Multiplatform   Beatmania
Arcade   Bishi Bashi Online
Famicom   Blades of Steel
W9x   Busou Shinki Battle Rondo
Game Boy Advance   Castlevania : Akatsuki no Minuet
Game Boy Advance   Castlevania : Byakuya no Concerto
X68000 PlayStation   Castlevania Chronicles
Arcade   Cue Brick
DS   Contra Dual Spirits
Arcade PlayStation   Dance Dance Revolution
Famicom   Doremikko
Famicom   Dracula
PlayStation 2   Dream Mix TV World Fighters
Famicom   Falsion
Super Famicom   Ganbare Goemon Yukihime Kyuushutsu Emaki
Famicom   Ganbare Goemon 2
PlayStation   Ganbare Goemon: Uchuu Kaizoku Akogingu
Arcade PlayStation 2   GuitarFreaks & DrumMania
Arcade   Hexion
Arcade PlayStation 2   Keyboardmania
Game Boy Advance   Konami Wai Wai Racing Advance
i-Mode   Konami Wai Wai Sokoban
Famicom   Konami Wai Wai World
Arcade   Kyukyoku Sentai Dadandarn
Arcade   Metamorphic Force
Famicom   Moai Kun
PlayStation 2   Neo Contra
Game Boy   Other Life: Azure Dreams
MSX   Penguin Adventure
Arcade PlayStation 2   Pop'n Music
Mega Drive   Rocket Knight Adventures
PlayStation   Speed King NEO KOBE 2045
Arcade   Super Bishi Bashi Champ
Famicom   The Goonies
Arcade   Trigon
Saturn   Tokimeki Memorial
PlayStation   Tokimeki Memorial 2
PlayStation 2   Tokimeki Memorial 3 ~Yakusoku no Ano Basho de~
PlayStation 2 DS   Tokimeki Memorial Girl's Side
Arcade   Vic Viper
Arcade   Violent Storm
Famicom   Wai Wai World 2 SOS!! Paseri-jou
Famicom   Yume Penguin Monogatari

Anime Cameo

Anime   Hayate the Combat Butler
Anime   La BlueGirl
Anime   Pani Poni Dash!
Anime   Sky Girls
Anime & trading cards   Yu-gi-oh

Moai Reference Only

Famicom   Knightmare II: The Maze of Galious
Famicom   Nazo no Kabe - Block Kuzushi
Arcade   Pandora's Palace
PlayStation   Poitter's Point and Poitter's Point 2
Mega Drive   Snatcher

Reverse Cameo

Special guest appearances of other Konami stuff in Gradius Games.

Super Famicom PlayStation Saturn   Jikkyou Oshaberi Parodius
Multiplatform   Gradius II GOFER no yabou

 

The Ghost of Horo Temple from Ganbare Goemon (Japanese, Super Famicom) (she's the first boss you fight) shows up as an end of level boss.

Goemon also shows up as an end of level boss.