Paco showed my how to do the egg trick in the Gold/Silver versions. It is very much like the Celebi trick without eggs, but with a small change. Here is how it is done:
1. Get an egg. If you want Celebi, the egg will have to hatch into a Pok?mon that knows Beat Up as its third move. One way to do this is to breed a male and a female Sneasel who know both know Beat Up as the first move.
2. Get a bad clone. I find the easiest way to get one is to deposit a Pok?mon in the PC, turn off the PC, save from the Start Menu, reset the game about a second after it says "Don't turn off the power."
3. Get any 5 Pok?mon you don't care too much about. Paco says it is important that they all know 4 moves each.
4. Make sure the egg is in the PC. The 5 other Pok?mon and the bad clone should be in your party.
5. Go to the Daycare and talk to the man. Ask him to raise the bad clone. Then take it back from him, and it will be a # 000.
6. Put 000 at the top of your party and go to the PC.
7. Using the "Move PKMN w/o mail" option, move any Pok?mon you don't care about from a PC box to the top of your party. Its name should become the name of its original trainer (OT). You will now have 7 Pok?mon in your party.
8. Go back to the Daycare man and ask him to raise your 000, then go back to the PC.
9. Deposit the first and second party Pok?mon.
10. Using the "Move PKMN w/o mail" option, move the egg from its PC box to the top of your party.
11. Deposit the 4 next party Pok?mon (not the egg).
12. Withdraw any Pok?mon that you didn't use for the trick.
13. Walk around a few minutes and the egg should hatch.
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Here is Paco's video of the trick:
http://rapidshare.com/files/52836138/Celebi_Egg_GS.zip.html------
As far as I know, the Celebi egg would be the only way to get the Celebi Pok?dex entry with only one GameBoy (and without a cheating device). However, it seems that the egg trick messes up the Pok?dex, so if we do the trick again to get a different Pok?mon, we might lose some Pok?dex entries. I am trying to work on a way to complete the Pok?dex in spite of that glitch. I'll let you know if it works.