Gyaos resembles a giant cross between a pterosaur and a bat, with a flattened, arrow-shaped head, leathery wings with three claws on each, taloned feet, and a fish-like tail. It has dull blue skin in most incarnations. The Gyaos in Gamera: Guardian of the Universe had a more streamlined appearance, with larger wings and a longer neck, as well as red eyes.
In Gamera vs. Gyaos, Gyaos appeared in Japan from a large cavern, and feasted on blood (principally that of livestock and human beings). Soon, Gamera confronted Gyaos, and after a battle, Gyaos was forced to flee and Gamera was forced into the ocean to recover. It is soon learned that light from the sun causes Gyaos' tissue to shrivel, so the light of the city stadium of Nagoya kept Gyaos at bay. Gamera soon returned to finish Gyaos off and, after a battle in the air, Gyaos cut off his own foot to escape from the sun. The protagonists developed a plan to place artificial blood on a rooftop in Nagoya. The plan was to keep Gyaos drinking the blood for so long that the sun would come up and kill him. But Gyaos proved more cunning than originally thought and used a strange fog attack to protect himself from the sun. The next plan was to lure Gamera to Gyaos' lair in the forest by setting the forest on fire. Gyaos used his fog attack to put out the flames, but Gamera arrived and eventually defeated Gyaos by throwing the beast into a volcano.
Space Gyaos appeared briefly in Gamera vs. Guiron, and quickly finds itself victim of Gamera's titular opponent. In reality, Space Gyaos is simply the Gyaos prop from Gamera vs. Gyaos painted silver. This was done because production crew of the film did not have the time and budget to invest the time of creating another new monster.
Stock footage of Gyaos was later featured in the Shōwa series recap Gamera: Super Monster among others.
In 1995, Gyaos was revived with Gamera for the first Heisei era Gamera film, Gamera: Guardian of the Universe, which follows a story similar to the one above. In this, the Gyaos were reimagined as creations of the ancient Atlantis civilization designed to control pollution. However, the asexual Gyaos reproduced out of control and destroyed their creators. As a last-ditch defense, they created the Gameras, who eliminated the Gyaos, although not in time to save the Atlanteans. However, clutches of Gyaos eggs survived and one clutch hatched in the modern world. After killing and eating all the inhabitants of Himigami Island and attacking the mainland, the Gyaos were lured to the Fukuoka Dome stadium in Fukuoka. The last Gamera, sensing his ancient foe, awoke as well and made his way to Fukuoka. Gamera attempted to stop the Gyaos, killing two, but was hindered by the misguided military, who considered him to be the greater threat. Eventually Gamera killed the last Gyaos, which had nested in Tokyo and had grown into Super Gyaos. Super Gyaos would also appear in flashback scenes in G3, and would eventually be revealed as the real killer of Ayana's parents.
The Gyaos do not appear in G2, but they are mentioned and are the subject of a book.
The Gyaos returned in a new, evolved form known as the Hyper Gyaos in G3, after Gamera used his mana cannon to destroy Legion in G2, which drained the Earth's mana, creating the right conditions for more Gyaos to hatch. Early in the film, Gamera battles a pair of Hyper Gyaos in the Shibuya district of Tokyo, killing them but also killing and injuring as many as fifteen to twenty thousand civilians. After Gamera defeated Iris in Kyoto, thousands of Hyper Gyaos swarmed towards Japan, intent on killing Gamera once and for all.
The Gyaos make a cameo appearance in the newest Gamera film Gamera the Brave, as Gamera dies while fighting several Gyaos in a small village. This makes them the only monster in the Gamera series apart from Gamera himself to have appeared in the Showa, Heisei and Millennium series in some shape or form.
In all incarnations the Gyaos have been shown to be astonishingly capable flyers, able to fly incredibly fast and perform agile aerial maneuvers with ease. Also, Gyaos have the ability to fire sonic beams from their mouths that can cut through nearly anything short of Gamera's shell, however, the Showa Gyaos had a weakness in that the double throats it used to create its Sonic Ray limited the mobility of its head and neck. Although they are nocturnal, the Gyaos can overcome the sun: emitting a fog-like gas to obscure the sun and douse flames, while the Heisei version simply evolves into Super Gyaos and grows protective plates over its eyes. While sunlight was merely an annoyance to the Heisei Gyaos, it was a fatal weakness in the Showa Gyaos, as Ultraviolet rays ate away his tissue. Interestingly, the Space Gyaos in Gamera vs. Guiron seems to have no such aversion to sunlight. The Showa version also has a regenerative ability, as it was able to regrow its severed foot after only an hour. The Heisei version showed several evolutions: it was asexual and could reproduce on its own. While not shown to be regenerative, the Super Gyaos is immensely physically tough, able to easily shrug off missile attacks, and even survive orbital re-entry however this isn't surprising as many kaiju have done things similar to these feats. In all eras Gamera and Gyaos are considered archenemies.
Gamera vs. Gyaos
Gamera vs. Guiron
Gamera: Super Monster (Stock Footage)
Gamera: Guardian of the Universe
Gamera 3: Revenge of Iris
Gamera the Brave