Use Game Shark.
There is no legal way to encounter Celebi in Gold, Silver, or Crystal. Supposedly there was a way to do it in Crystal Version in Japan, it involved a gadget that connected the Game Boy to a cell phone, but we never got that here and the relevant ingame features were taken out.
If there is a "Celebi Glitch" similar to the Mew Glitch discovered for RB, we don't know of it yet.