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 immediately ordered him to use a pebble to stop the aquarium filter. With the filter sabotaged, he assumed the dentist would remove the fish to clean the tank, giving them the opportunity to return to the ocean.
Eventually, Gill helps Nemo escape the office by jumping onto 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.
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 for 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, on 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.
"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.