Instructions
Collect all 250 Pokemon found in the game. A majority of the Pokemon can be obtained by walking in tall grass and any other location that wild Pokemon can be encountered; however, some Pokemon can be captured only by completing side quests for the game's many citizens. Once you have captured all 250 Pokemon, you will receive instantly the National Pokedex, displaying that one Pokemon can still be captured.
Trade Articuno, Zapdos and Moltres from your game to another player. If any of the three Pokemon are still in your possession, whether in your party or in your PC Box, you will not be able to receive the GS Ball. Trade Pokemon by going to the top floor of the Pokecenter and talking to the lady behind the first counter.
Travel to battle station in Viridian City. This city will not be unlocked until near the end of the game. It is located on the western side of the continent Kanto. Once at the battle station, a woman will be waiting near the exit.
Talk to the women waiting by the exit. After a brief dialog, she will give you the GS Ball, which then can be used later to catch Celebi. When you receive the item, you also will receive a side quest that will direct you and give you instructions for catching Celebi.