NiGHTS into Dreams
"This fish-type Second-Level will swallow anything up completely with his huge mouth. His form is ferocious, but his personality is surprisingly calm. Knowing his weakness is his slowness, he’s set up traps all around his body to catch his prey."
- Gulpo, according to a translated japanese bio.
The player must use red fishes pointing to a specific direction which will launch NiGHTS into the direction they are pointing, the player will need to find the right fishes pointing at Gulpo to damage him. Sometimes, Gulpo will surround himself with dark energy orbs and shoot them at the fishes, making them get surronded by a dark energy for a few seconds which will damage NiGHTS if he tries to use them, the player also must be careful and not launch NiGHTS into Gulpo's mouth, otherwise he will take damage, getting damaged by any of Gulpo's attacks will teleport NiGHTS back to the outside of the tank. Sometimes, Gulpo will turn upside down after getting hit, attacking Gulpo in this state will give him more damage. It's possible to defeat Gulpo in three hits. 
After the battle, Gulpo isn't seen again.
Gulpo's battle takes place in the deeps of the ocean around a transparent and cylinder tank surronded by fishes.
NiGHTS Archie Comics
In the Archie Comics series, Gulpo makes a small appearance only in the second issue. Claris was lunching at the Twin Seeds Museum with Roger, who was dualized with Reala, Reala was seeing everything through Roger's eyes and noticed that Claris felt disgusted over a fish identical to Gulpo on her meal.
Later, a Shleep makes Claris fall asleep and she is transported to a giant aquarium, where she is attacked by Gulpo, Claris screams for help and NiGHTS, who was dualized with Elliot and fighting Jackle, goes assist her, unfusing and leaving Elliot with Jackle alone.
NiGHTS and Claris start fighting Gulpo, but NiGHTS knows he has to come to back to help Elliot, NiGHTS eventually leaves Claris with a message on the glass, telling her to sing, when Claris does that, the aquarium's glass breaks and Gulpo can't fight anymore as there's no more water for him to swim, as Claris leaves the aquarium, Gulpo starts shrinking.
After Claris is gone, Clawz appears and eats Gulpo alive. Unlike the other second level nightmarens, Gulpo doesn't make another appearance. It's unknown if Gulpo could come back or be revived as he was eaten by Clawz, however, considering Puffy was crushed against the wall and later reappeared, it's possible that he could.
- Christmas NiGHTS into Dreams: Gulpo's render appears in the concept art.
- Sonic Pinball Party: Gulpo made an appearance in Sonic Pinball Party as a boss, curiously, instead of being the boss of the Mystic Forest table, he is the boss of the Frozen Bell table. To defeat him, the player must make the ball paraloop him enough times under 180 seconds.
- Sonic Lost World: Gulpo appeared as the third boss of the Nightmare Zone DLC of Sonic Lost World, where he is riden by Master Zik. Gulpo will chase Sonic across the stage and shoots spike balls at him, to defeat Gulpo, Sonic must make him fall into a hole.
- In Burning Rangers, there's a giant mechanical fish that resembles Gulpo in some bits.  It's possible that Gulpo's 3D model was used to create it.
- In the NiGHTS into Dreams game files, Gulpo is nicknamed "M6:MONSTER-FISH".