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.
Fan Nickname: "Kenny/Kendra/Kennys" for all the child characters who's actual names the audience don't bother to remember. In two cases they literally were named "Kenny" though.
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.
Name's the Same: Daiei's Barugon / Toho's Baragon. The two monsters themselves are quite different.
Gamera wasnt Daieis 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 the aforementioned side effects and one rat growing to "mammoth size", at the climax the rats would eventually turn on and eat each other. The movie was canceled primarily because someone decided to use actual sewer rats, which were not only untrained but infested with parasites.
Gamers 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 monster 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.