Gradius/WMG

Gradius Gaiden's Loop 2+ Stage 8 endboss, called "Heaven's Gate", is the Heaven's Gate cult.

For one, Gaiden was released in August 1997, five months after the Heaven's Gate mass suicide. Also, if you read up on HG's beliefs, its members believe that the mass suicide that they would eventually commit would take them onboard a spaceship. So when you reach the last boss battle of stage (>1)-8, the boss you fight is the very spaceship that the members suicided their way onto, hence its name "Heaven's Gate." Nice Job Killing A Cult Hero.

Gradius Gaiden's Final Boss, called "O.V.U.M", is Bacterion who has become Giygas.

Just before you fight O.V.U.M, you see a red image in the background before the background changes. That is Bacterion in his "Giygas" form. After the background changes, 3 Intruders (the dragon boss from Salamander) fly from the background into the foreground to form Zelos, which change into Brain Golem, which changes into a small sphere that can be destroyed. So Bacterion materialized himself to battle you, which didn't work.