There are two ways to access the Glitch City: a Walk-Thru-Walls code and the glitch pokemon ?????. The Walk-Thru-Walls code can only be done in Pokemon Crystal and it seems that the ????? code can only be done on Gold and Silver.
To access the Glitch City through ?????, use one of the Gameshark codes for Pokemon Gold and Silver:
01FDEDD0
01FEEDD0
01FFEDD0
Defeat the wild pokemon that appears and you will be in GSC Glitch City.
Like in Pokemon Red, Blue, and Yellow, you won't be able to move around much without a Walk-Thru-Walls code. There are Pokemon Trainers from the area you battled ????? in this version of GSC Glitch City, but they will usually freeze your game. If you do battle the trainer, the trainer will look like a ?????.
For the walk thru walls codes for Crystal, they are:
0100FAC2
0100FBC2
0100FCC2
0100FDC2
Walk east off the area outside and you will enter GSC Glitch City. If it freezes your game, try a different place. You will enter a Glitch City at night.
There are much more glitches resulting from it.
First, it changes some of your party's Pokemon names a bit slightly by replacing two letters with ?9. And it also changes the OT's name in some Pokemon.
Then changes are made to your character's stats screen. It adds a semicolon to your money and removes some badges.
Possibly the biggest change is your pack. It glitches up the number of items and even changes what items you have!
There is a glitched trainer in GSC Glitch City. It's often either a girl or your character. He/she can see diagonally and if he/she does see you, then that person will speak messed up text or crash your game. If they do speak glitch text, you will be taken into a battle and the text box will fill up with question marks. To find the glitched trainer, press Enter and look at your Pokemon, and you will enter a Glitch City at daytime and with different tiles. But your character might be invisible.
You can also do this indoors! You will be in a room where flowers are sitting on the chairs if there were any chairs in the original room. And some objects are "glowing" in the dark. Press Enter and you will be in a indoors-style GSC Glitch City.