Location: Ilex Forest
To acquire a GS Ball you have to connect the game to the mobile phone with the Pokemon Mobile Phone System GB. This cannot be done in the US. You must have all 16 badges, beat a minigame, and complete a quiz to have the attendent at the Pokemon Communication Center in Goldenrod City give you a GS Ball.
In the Virtual Console release, you will receive the GS Ball at the Goldenrod City Pokémon Center after you have defeated the Elite Four, without needing to go through Kanto.
Once you have the GS Ball, take it to Kurt to have him inspect it. After the turn to the next day he'll tell you about a strange presence in the ball before dashing outside. Follow him and he'll say Ilex Forest is uneasy and that you should check it out with the GS Ball.
The trees will be rustling in the forest. Take the GS Ball to the shrine. When asked if you want to put the GS Ball in the shrine, say yes and Celebi will arrive to challenge you.
Celebi holds no special items. Grass/Psychic is weak to Fire-, Flying-, Ghost-, Dark-, Ice-, and Poison-types and has a double weakness to Bug-types but offers good STAB and learns powerful moves like Giga Drain, Psychic, and Solarbeam. Because you need all 16 badges to fight it but is faced at a low level of 30, it would be wise to bring something that can inflict status before it uses Safeguard but not OHKO it.