NiGHTS into Dreams
"By Second Level standards, he’s got a huge, enormous body but he’s really just a big air-head. The moment he considers you an enemy, he’ll stop at nothing to chase you down. He possesses the ability to re-grow his head from his long tail."
- Gillwing, according to a translated Japanese bio.
In NiGHTS into Dreams, Gillwing is Spring Valley's boss and a Claris nightmaren, who'll attack after all ideyas are collected in the level. His boss battle theme is called "The dragon gave a loud scream".
Gillwing's battle is known for being the fastest in the NiGHTS series, all he does is constantly fly in circles and if NiGHTS run into his mouth or tail, he will get damaged. To defeat him, it's necessary to destroy his whole tail and the player can do that in a fraction of seconds by going behind him and paralooping the end of his tail tail, the player must be careful because they may touch his tail when trying to do that, which will damage NiGHTS. 
A slower, but safer way to defeat Gillwing is by simply grabbing and tearing off his head, which will regenerate and replace a part of his tail, Gillwing will start shrinking and once there's no more tail, he will not be able to regenerate and will be defeated, however, the player should not fly into his mouth, otherwise, it will damage NiGHTS. 
It should be noted that the tail's hitbox size has changed in each version of the game, in the Sega Saturn version, it's much smaller, but in the Ps2, Ps3, Xbox 360 and PC versions, it became much bigger, meaning paralooping Gillwing's tail is much harder and risky in these versions.
After the battle, Gillwing isn't seen again.
Gillwing's battle happens in a weird garden like environment with giant flowers which are constantly opening and closing, mushrooms which keep growing and shrinking, rocks floating around and two red suns, in the center of the area, there's a weird tree with a blue star spinning above it, it's possible that the garden is located in space or perhaps in another planet, due the dark sky, floating rocks and the two red suns.
Christmas NiGHTS into Dreams
Gillwing returns to the promotional sampler disc of NiGHTS into Dreams, Christmas NiGHTS into Dreams, where he is once again the boss of Spring Valley (both Claris' and Elliot's versions of the level). In this game, Gillwing is keeping the Twin Seeds christmas tree star on his tail.
The battle is identical to his NiGHTS into Dreams version, where he will just fly in circles and the player can defeat him by paralooping the end of his tail or by grabbing and tearing off his head until he run out of tail. In Claris version of Spring Valley, Gillwing will start attacking from the left while in Elliot's version, he will start attacking from the right.
After defeating him, the star on his tail is brough back to the christmas tree.
The environment where the battle takes place is similar to the one from NiGHTS into Dreams. Some flowers are replaced by Christmas decorations and presents, the red suns are replaced by purple nebulas, there are gems spinning around the tree in the center of the stage and after the fight, a giant christmas tree appears in the center.
The star at the top of the Nightmaren tree-like formation in Gillwing's realm in NiGHTS into Dreams has use in Christmas NiGHTS into Dreams as after defeating Christmas Gillwing the player is awarded with the star that will eventually sit atop a very tree-trimmed Twin Seeds tower. This will make the player win the game in Christmas NiGHTS, but they do not recieve the star at the end of the battle in the original game.
NiGHTS Archie Comics
In the Archie Comics series, it's Elliot who fights Gillwing, contradicting with the game, where he is a Claris nightmaren. In the first issue of the series, Wizeman releases Gillwing after seeing Elliot dualizing and releasing NiGHTS from the Ideya Palace. NiGHTS and Elliot were about to fight him when Claris, who was being attacked by Puffy screamed for help, NIGHTS unfused with Elliot to assist Claris, leaving him alone with Gillwing, as Elliot couldn't defeat him by himself, he ran into an Alarm Egg to wake up.
Elliot comes back to fight Gillwing in the second issue and defeats him off screen, Jackle later warns Wizeman about the news. Gillwing later makes one last appearance in the fifth issue, where he tries to fight Reala along with Puffy, Jackle and Clawz after Reala proclaiming himself the new ruler of nightmaren, before Gillwing or any of the nightmarens have a chance to fight Reala, the later attacks and makes them fall into an abyss.
- Sonic Adventure and Sonic Adventure DX: Gillwing made a cameo in Sonic Adventure and Sonic Adventure DX as a card in the NiGHTS thematic pinball table from Casinopolis.
- Sonic Pinball Party: Gillwing appears in Sonic Pinball Party as the boss of the Spring Valley's pinball table, to defeat him, the player must make the ball paraloop him enough times under 180 seconds.
- Sonic and All Stars Racing Transformed: In Sonic and All Stars Racing Transformed, Gillwing makes a cameo in the first lap of the Dream Valley circuit when the racers are transported to Nightmare. Additionatly, a NiGHTS sticker set, which included a Gillwing sticker, was made for the game, but it got disconsidered. 
- Sonic Lost World: Gillwing appeared as the sixth boss of the Nightmare Zone DLC of Sonic Lost World, where he is riden by Zavok. Gillwing will slide across the stage and shoots fireball at Sonic, to defeat him, Sonic must attack a red orb at the end of his tail enough times until he gets defeated.
- In some of the earliest concept arts of the game, there's what appears to be a squid like nightmaren who was most likely one of Gillwing's earliest designs, as he has similar color patterns.
- His name comes from the fact he has wings where tadpole's gills are located.
- Gillwing is Frozen Bell's boss in the Beta version of the game.
- The third level nightmaren Party, looks a bit similar to Gillwing design wise, though with more feminine traits and different colors.
- Also just like Gillwing, Party's tail is it's main weapon.
- He is one of the two second level nightmarens who can be defeated with a single attack, the other is Queen Bella.
- Gillwing is the only villain besides Wizeman to have been the main antagonist of a NiGHTS game, being the main antagonist of Christmas NiGHTS into Dreams.
- In the NiGHTS into Dreams game files, Gillwing is nicknamed "#M1:FROGED-DRAGON".