Gill's only dream is to escape from the tank. He has made several escape attempts, but the first time, he landed in the dental tools, giving him severe scars on his right side, and destroying his right fin (similar to how Nemo has a tiny right fin). His appearance is modeled after his voice actor, Willem Dafoe, especially the coloring around his mouth to show the deep wrinkles that Dafoe has on his face.
He is the leader of the fish living inside the dentist's office fish tank called the Tank Gang.
Unlike the other Tank Gang members, Gill had a neutral reaction to Nemo's entrance. Upon realizing that Nemo could fit through small tubes, Gill had an idea: since Nemo was the only one who's small enough to enter, he had to use a pebble to block the aquarium filter, which will quickly cause the water to get dirty, forcing the dentist to put everyone in plastic bags, and then they'll all escape to the ocean, and Nemo accepts.
However, something goes wrong: the pebble gives away, causing Nemo to be sucked into the gears, and he almost dies, although the other fish save him by pushing a plant into the tube. Gill, shocked and upset about the fact that he almost sacrificed the little clownfish, isolates himself, feeling guilty for his failure and for almost killing him with his escape plan, and later, he apologizes to him for putting him in danger. However, when Nemo hears his father's amazing achievements, he tries again, and Gill reacts with horror, especially after hearing a terrible sound of locked gears, but when he realizes that Nemo survived and has successfully blocked the filter, he is very happy of him.
The tank soon gets dirty, but the dentist, during the night, installed a new filter, the AquaScum 2003, which cleaned it, and then captured Nemo with a net, but everyone saved him by swimming downwards, which Nemo learns to use later on, although he's then put in a plastic bag and his attempt to escape through the window failed, so he fakes his death. Gill is initially horrified, along with the rest of the Tank Gang, but when the little fish winks at him, he realizes that it was a trick, and then helps Nemo escape the office by jumping onto Darla's head and then on the dentist's mirror to catapult the clownfish into the air so that he lands into a sink leading to a drain, which leads to the ocean. After Dr. Sherman puts Gill back into the tank, the Tank Gang congratulates him on finally succeeding in helping Nemo return to the ocean and they all gladly watch Darla break the sink Nemo went through and get harshly sprayed in the face.
At the end of the film, Gill and the other Tank Gang members manage to escape from Dr. Sherman's office. Since Dr. Sherman's new AquaScum 2003 has broken, the fish tank has become filthy again, and now he has to take his fish out, put them in bags, and clean the tank himself, and while he complains to Barbara about the device breaking when it has a lifetime guarantee, Gill and the others make their way out of the office. They are seen having crossed the street and jumped into the ocean to their freedom, although still trapped in the plastic bags (they presumably managed to get out by the ending credits).
Gill, along with the rest of the Tank Gang, appears in the post-credits scene. They are still trapped in their respective bags, now covered with algae, and then captured by the Marine Life Institute volunteers.
Gill's voice, as Dafoe said, is based on the character Earl Copen, whom he plays in the movie Animal Factory.
Gill's species, Moorish Idols, are notoriously difficult to keep in captivity due to their diet and mannerisms. This may be an interesting way to show Gill's deep desire to escape from the aquarium and into the ocean. In addition, Gill was pressuring Nemo to follow his challenging orders on his escape plan.
His Polish name is "Idol" (from his species) because Gil means "bullfinch" or "mucus" in Polish.
There is a deleted scene as an extra in the Audio-Visual Commentary on the DVD, wherein Gill says that he has brothers and sisters named Marco, Polo, Lester, Linus, and Lulu. He also tells Nemo that he is from a place called Bad Luck Bay, a "rough part of the ocean" that is "shaped like a human's skull." Later, a program comes on TV, where a man reads a book to his child about Marco, Polo, Lester, Linus, and Lulu's adventures in Bad Luck Bay, and Nemo finds out that Gill was lying to him.
Gill is named after the respiratory organ found in many aquatic animals, especially fish.
"I was so ready to get out, so ready to taste that ocean, I was willing to put you in harm's way to get there. Nothing should be worth that. I'm sorry I couldn't get you back to your father, kid."
Gill in Disneystrology book
Gill's escape plan finally succeeds...except they're still stuck in plastic baggies, with no idea how to get out of them.