Actor Allusion: The scene near the end of Gamera vs. Guiron with Kondo's glasses falling down his nose was a reference to a trademark pose the actor, Kon Omura, did in his comedy routines and ads. Since these all aired in Japan and he is very obscure outside of his home country, most people outside Japan just tend to see the scene as bizarre.
Bad Export for You: Until recently with the release of the whole Showa series by Shout Factory, most Region 1 Gamera DVD releases contain deteriorated fullscreen 16mm TV prints, most of them which were actually stolen from the video catalog Sinister Cinema.
Dueling Dubs: About half the series was dubbed twice: First, by AIP-TV (The English Language Dubbers Association and TitanProductions did the actual dubbing) in the '60s and '70s, then later by Sandy Frank in the '80s, who used pre-existing Hong Kong produced dubs for at least Barugon and Gyaos. The first set of dubs are the ones that show up on budget DVD releases (the actual pristine film elements are currently stored in the MGM vaults); the second appeared on MST3K.
Follow the Leader: The Godzilla series, of course. Although it's worth mentioning that after Gamera started to catch on, the Godzilla series also began featuring children more prominently than it ever had before, and eventually, gore into the monster battles as well (The Godzilla films of the 1970s frequently had Godzilla bleed, something that had never happened in the series before). Some would also say the decision to utilize heavy amounts of stock footage in later Godzilla films such as All Monsters Attack, Godzilla vs Gigan and so on, was also inspired by the Gamera series' successful use of such.
Keep Circulating the Tapes: Neptune Media's OOP tape of Gammera The Invincible is the only way the see the American theatrical version in widescreen. Shout! Factory intended to include this version on the Gamera DVD, but the UCLA, the owners of the print used for the Neptune tape, refused to offer it again for a new digital scan. Since Celebrity/Just For Kids never issued it on Laser Disc, the Hong Kong dub of Barugon, which was oddly not included on Shout! Factory's DVD, also shares this fate.
Science Imitates Art: Two species of extinct freshwater turtles are named after the eponymous flying turtle monster. Sinemys gamera, which had large, wing-like projections from its shell, and Gamerabaena, from the Hell Creek Formation (so it would've coexisted with dinosaurs like Tyrannosaurus and Triceratops).
Gamera wasn’t Daiei’s first idea for a kaiju movie, the first idea was a movie called Giant Horde Beast Nezura, which involved a bunch of sewer rats who mutate to human size after eating an artificial food stuff called "S602" which was disposed of because of a side-effect that made organisms that eat it grow in size. One of the rats would have grown to a truly mammoth size, and in the climax all the rats would turn on and eat each other. The movie was cancelled after the head of Daiei decided that the project was more trouble than it was worth: initially they used store bought rats, but found that they were too tame and wouldn't rampage through the miniature set. Then they moved to acquire actual sewer rats which proved to be even worse, they were not only untrained so they ended up destroying the set in a way they weren't intended to, but they were also ridden with parasites which became a health concern for the entire town. However, the staff at Daiei weren't to be dissuaded by this, and immediately began work on what would eventually become Gamera.
Since Daiei still had an unused suit for the Mammoth Nezura, Noriaki Yuasa was given permission to use Nezura in a Gamera movie. Obviously he never did.
At one point there were plans to revisit the concept with the addition of some of the rats growing wings and becoming "Nezoorabats".
Gamera vs. the Ice Men, planned as the second film in the series, would have Earth invaded by ice-based aliens who tried to create a new ice age and force humans to migrate underground while the invaders claimed the surface. When this failed, they intended to just enslave the human race instead, but Gamera, having escaped from the rocket he was imprisoned in, returns and battles their "Ice Giant" monster, which would have been modeled after the Frost Giants of Norse myth. The concept of an ice monster was subsequently incorporated into Barugon, and the "elemental giant" was reworked into the titular yokai of the Daimajin series.
Gamera Vs. Viras would have featured another monster, the mushroom-like Marukobukarappa, along with Viras.
Marukobukarappa was also going to be used in the unmade Gamera Vs. Garasharp.
The monster that Guiron fights when introduced in Gamera Vs. Guiron was originally going to be a horned, flying squirrel-like kaiju named Monga, but budget constraints prevented them from making two new monster suits so they instead spray painted the Gyaos suit silver and had Guiron fight a Space Gyaos.
There were plans for a "Destroy All Monsters" equivalent, in which the Zigrans and Virasians returned, now working together and using Gamera's DNA to create the two-headed Wyvern, a fire-breathing dragon monster (Daiei's answer to King Ghidorah) and using it and a flock of Space Gyaos to attack Earth. Gamera would be seemingly killed by Wyvern, but be revived by a benevolent alien race, along with his old enemies Gyaos, Barugon and Jiger, and a giant "whale god" (apparently the titular creature from the standalone film Kujira Gami) to join forces against the alien invaders and their monsters.
Wyvern was also featured in "Gamera Vs. Wyvern" (or "Gamera vs. Two-Headed Monster W"), apparently featuring just Gamera and Wyvern. The concept was subsequently reworked into Gamera Vs. Garasharp.
Gamera vs. Garasharp featured a massive, cobra-like creature, armed with a sonic rattle and poisonous breath, as the titular antagonist. The story ends with Gamera defeating Garasharp (naturally) and two babies emerging from her body. Just as the military closes in to destroy them, Gamera rescues them and flies them off to a deserted island, because he's the friend to all children, not just human ones. The film was scheduled for a 1972 release, but Daiei went bankrupt before they could start on it. (Garasharp would later be featured in the manga "The Last Hope", released in 2017 as a prequel to the Heisei Gamera trilogy; it was reissued in 2018 as a canonical installment of the series.)
Daiei had approached Toho with a proposal for a crossover film with Godzilla, but the latter studio rejected the proposal. Ironically, Toho would purchase the distribution rights for the Heisei Gamera trilogy.
Yoshimitsu Banno, director of Godzilla vs. Hedorah, had planned a 40 minute 3D Imax movie in which Gamera dealt with his own pollution monster and would have teamed up with a monster based off of Journey To The West's Sun Wukong.
Before Gamera: Guardian of the Universe was made, one plan was for "Gamera Vs. Phoenix", in which Gamera fights a giant fire bird called Phoenix. It wound up as a novel instead.
One draft iteration of Gamera: Guardian of the Universe sees Gamera face an Atlantean-derived Barugon. While this was ultimately dropped in favour of the more popular Gyaos, Kadokawa revisited the concept and turned it into a manga that formed part of the Heisei canon, fitting between Attack of the Legion and Revenge of Iris.
While designing the suit for Gamera in Revenge of Iris, they at one point considered having the plastron permanently opened after he used the Mana Cannon in Attack of the Legion. They also considered having him reuse that attack as the finishing move against Irys.
According to Shusuke Kaneko if Revenge of Iris had grossed one billion yen they would have begun production on another sequel.
The idea of a fourth Heisei Gamera film was eventually realized in the form of a fan made sequel called Gamera 4: Truth directed by independent filmmaker Shinpei Hayashiya.
Originally a squid kaiju was planned as the villain for Gamera: the Brave but they thought that the monster would be too similar to Viras, so they changed it to the reptilian Zedus.
An anime based on the franchise was planned (rumor was that Cartoon Network was going to sponsor it) but was scrapped after Gamera: The Brave tanked at the box office in Japan.
in which Gamera dealt with his own pollution monster and would have teamed up with a monster based off of Journey To The West's Sun Wukong.