From Bulbapedia, the community-driven Pokémon encyclopedia.
The GS Ball (Japanese, Pokémon Crystal: ジーエスボール, anime: ＧＳボール GS Ball) is a special and mysterious Poké Ball. It is colored gold and silver and has the letters "GS" inscribed upon it. GS may stand for Gold and Silver, as those were the first two games of Generation II.
In the games
In the Japanese version of Pokémon Crystal, players could use the Pokémon Mobile System GB to obtain a GS Ball as an event item which they could take to Ilex Forest. Using it at the forest's shrine would prompt an encounter where a wild Celebi could be captured. However, no equivalent event was ever offered to players of the English version of the game, thus rendering the GS Ball unusable, and under normal circumstances, unattainable. A modified version of the Celebi Egg glitch will spawn a GS Ball as a held item.
In the anime
The GS Ball in Professor Ivy's Lab
In the anime, Professor Ivy was the first character to possess the GS Ball. She called upon the help of Professor Oak to study it, but couldn't figure out how to open it. Ivy tried to open it with hacksaws, hammers, crowbars, power drills and lasers, but to no avail. At one point, Ash tried simply calling a Pokémon to be released, but this did not work. The ball also couldn't be transmitted via PC.
In Pallet Party Panic, Professor Oak tasked Ash with retrieving the GS Ball from Professor Ivy's laboratory, located on Valencia Island in the Orange Archipelago, and bringing it back to Oak for him to study.
Ash obtained it two episodes later, in Poké Ball Peril, and updated Oak on his progress. He put off completing the errand in the next episode so he could compete in the Orange League, a move which Oak approved.
In A Way Off Day Off, Ash compared it to a standard Poké Ball while everyone else was sleeping.
After a brief mention by Misty in The Underground Round Up, Ash finally gave Oak the GS Ball after returning to Pallet Town in A Tent Situation, but Oak put it away to examine later. The GS Ball was the last sucked in by Team Rocket's Super Sucker. When the trio's balloon was torn, the GS Ball was rescued with all the other Poké Balls.
In The Rivalry Revival, Oak's studies on the ball proved to be extremely difficult, with Tracey even admitting that no new information was gathered. Oak gave it back to Ash, asking him to deliver the GS Ball to Kurt of Azalea Town in Johto, a Poké Ball researcher and manufacturer.
In Once in a Blue Moon, Ash took the Ball out to polish it when it is stolen by a Quagsire. Ash, Misty, and Brock chased after the Quagsire before getting the Ball back by the end of the episode.
In A Shadow of a Drought, the trio arrived in Azalea Town, and Misty reminded Ash about the errand. The delivery was made in Going Apricorn!, and Ash broke the news to Oak in A Farfetch'd Tale. Its last appearance was in a flashback in Hour of the Houndour.
In an interview with Masamitsu Hidaka in 2008, it was explained that the GS Ball was originally going to contain a Celebi that was to be the star of a large arc of the Johto saga. However, it was decided that Celebi would be the star of the fourth movie, so the story arc was viewed as redundant and shelved. The ball was left with Kurt with the hopes that viewers would eventually forget about it.
For every episode of the anime's fourth season, Pokémon: Johto League Champions, three GS Balls are seen alongside the title card and the name of the episode. Each Ball is situated on a corner of the title card; there are two GS Balls on the two bottom corners and one on the top right corner.
This contradicts what was revealed by the aforementioned interview with Masamitsu Hidaka, that the Ball was meant to be forgotten, as the fourth season aired after the last mention of the GS Ball in A Farfetch'd Tale and its last appearance in Hour of the Houndour.
In the manga
In the Pokémon Adventures manga
The GS Ball was created by the Masked Man using a net made from the feathers of Lugia and Ho-Oh to capture Celebi. Only the feathers of the two legendary Pokémon could contain Celebi, and prevent their holder from suffering the ill effects of time travel. Gold was able to use his Pichu to destroy it and set Celebi free.