GS Ball Bulbapedia
The GS Ballis 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 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.