WMG / Gradius

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.

Vic Viper doesn't get destroyed when it gets hit/crashes into something

The temporal space fighter is simply equipped with some sort of proximity-triggered failsafe that warps it away to a previous safe spot before it is destroyed or serious damage is done, the trade-off being a high energy cost (lives and power-ups). This would explain the Vic viper's memetically fragile nature, as well as why the explosion looks like the ship zapped out of existence.

The pilots of the games are Newtypes/Cyber-Newtypes.

Let's face it, how does one explain all the crazy shit you can pull off with those Options, especially in the opening cutscene of V? The pilots possess exceptional reflexes in order to survive their treks, the Options are the Bit/Funnel equivalent for the ships, and the original Vic Viper pilot, James Burton, and its designer, Dr. Venom, possessed ESP (similar to Newtypes); thus giving them a method to use the Options the way they do. This carried on to James' descendent, David, who also possessed ESP, survived his mission, and did crazy stuff with his ship's Options. Any other pilot would have had to been enhanced, akin to creating Cyber-Newtypes; hence, how Lord British and the rest of the pilots can use their Options and have lightning-quick reflexes.

Gradius V Stage 1's boss is being piloted by the moai, or the spirits thereof. Furthermore, the moai were driven to extinction.

It would explain why said boss shoots rings that can be shot away, and why V does not have a moai stage.
  • JOSSED! Moais still exist in NEO, but V avoided this by not going to vandalize them.

Gradius V and Gradius NEO are Continuity Reboot of the franchise.

Since Gradius V is set in the same millennium as Gradius NEO (both set thousands of years after previous Gradius games) It would be safe to assume that the Gradian Empire doesn't even exist anymore prior to the events of Gradius V. Also, the Lars Empire is behind the Bacterians who was destroyed in Gradius V.

Galamoth, Red Falcon and Black Viper are pieces of Bacterion.
Bacterions are dark, "demonic" creatures, Red Falcon is "Emperor Demon Gyaba" in Japanese, Black Viper presumably has a similar title like in English, and Galamoth (see WMG.Castlevania) is obviously an alien. Galamoth didn't appear till 1796 (7807 Gradius year) and Red Falcon didn't appear till 2633 (8644) and Black Viper two years later, both well after Gradius IV, where Gofer broke himself into pieces. Some of the pieces could have landed on Earth as Galamoth and later Red Falcon and Black Viper.

The Bacterians came into existence through a Stable Time Loop
In the plot of Gofer No Yabou Episode II/Nemesis III, the Bacterians travel back in time about 15 years before the events of the original game. Because the player travels back after them and destroys Gofer II in the past, pieces of him spread throughout the universe. While it's unclear where the Bacterians truly originated, this loop ends up resulting in the events of the first game and beyond.