The GS Ball is a mysterious Poké Ball that Professor Ivy found in the Orange Islands. The Poké Ball got its name from the GS looking symbols on the upper golden part. Ash was given a secret mission to deliver it to Professor Oak from Professor Ivy, and then to Kurt.
The GS ball is a normal sized Poké Ball that doesn't seem to shrink like normal Poké Balls. Like most Poké Balls it is split horizontally down the middle into two different colors; Gold and Silver, with G and S shaped symbols on the upper golden part. It also has the black middle line separating the upper and lower parts with the button in the middle.
Later announced by Masamitsu Hidaka that the GS Ball stood for Gold/Silver ball and was suppose to contain Celebi and was to follow Ash Ketchum around in the Gold/Silver series, however they decided at the last minute to save Celebi for the Pokémon movie "Pokémon Forever", thats why they got rid of the GS Ball and gave it to Kurt to never be seen in the series since.
So far the GS Ball's history is unknown however it is said to come from the Orange Island where Professor Ivy found it. It is however similar to the inscriptions in the ruins of Pokelantis and Pokemopolis, Ho-Oh and Lugia also seem to be related to the GS Ball mystery sense the GS seems to be standing for Gold which represents Ho-Oh, and S for Silver for Lugia. Lugia was also in the Orange Islands at the time of the GS Ball.
Professor Ivy's Discovery
Sometime when Ash started his journey Professor Ivy discovered a mysterious Poké Ball and started to study it, she named it the GS ball since it had G and S looking symbols on the golden part. They were also unable to break it using heavy equipment, and found out little more than that it was indestructible.
Ash's Secret Mission part A
Professor Oak sent Ash to the Orange Islands to bring him the GS Ball for study. Brock stayed behind with Professor Ivy and Tracey took his place.
Enter into the Johto Journeys
Professor Oak couldn't do much about the GS ball and sent Ash to the Johto region to where his grandson went on his next adventure. Ash was to give the Poké Ball to Kurt a Poké Ball maker.
The Quagsire Theft
In between finding Kurt and getting his mission to go to Johto from Professor Oak, the GS ball is stolen by a Quagsire and ends up being the last thing to float down stream, which was in the local's superstitions: Lucky.
Ash's Secret Mission part B
Ash finally makes it to Kurt and battles the Gym leader to get his next badge. Kurt did some tests but all show inconclusive, and couldn't figure out what it was and said he'd keep it a bit longer to study.
The GS Ball's current situation is unknown and it still remains a mystery, it is hinted to play an important role in a future Pokémon event. However, it is likely to never play significance, as the original idea of a Celebi appearing in the ball that would have been a major story in the Johto saga even was retconned, since Celebi starred in the fourth movie instead.
Some people think gs stands for gold and silver as those are the colors on the ball.