Red Gyarados Needed and ability to trade to Generation 1
First, trade the Red Gyarados to a Red or Blue version, teaching it the Mimic TM, and trade it back. Then, using the move deleter in Blackthorn City, delete all its moves except Mimic. Then, trade the Gyarados back to the Red or Blue version and battle a Ditto. While battling the Ditto, get Gyarados to use Mimic after the Ditto uses Transform. Gyarados can then use Transform. It then transforms into Ditto, after which Ditto transforms into the transformed Gyarados. After that, catch the Ditto, which will be shiny when traded to Gold, Silver, or Crystal.
Needs Shiny Gyarados traded to R/B/Y
Another, even more complex trick involves restarting the game; it does not always work, so it is dangerous to do. The player takes the name ??w?x?y (case sensitive), then uses a combination of the Missingno. trick and the aforementioned Shiny Ditto trick. Before doing this, you must look at a Pokédex entry for Ditto. Once this is done, the Shiny Ditto trick can be used on a level 230 Missingno, which will only know Transform. The Missingno. must be level 230 for this to work. When the Missingno. is caught, it will have the DVs of a shiny Pokémon. When traded to a Gold or Silver version, if the game allows it (sometimes it may say the Missigno. is unusual and cannot be traded) it will be a Shiny Pokémon. If the "ghost" form of Missingno. is traded properly, it becomes a Shiny Lugia.
To get to Glitch City, you have to hear the cry of a Machop then use the Coin Case, if done correctly then the game will restart and all the colours will be wrong, it is not known what happens if you save here so try it at your own risk, the cry of the Machop can be the one in Goldenrod Mart or Vermillion City. Doing this with most other Pokémon will cause the game to crash but Ratticate makes it say "Which move?".
To get more phone calls, talk to your mom and continuously flick back and forth with Daylight Saving Time.
Warning: Use at your own risk, some of these cheats may cause data loss or data change
First you go to a Pokémon center and save the game. then you place the Pokémon in a box, and then go to change box. when the overwrite screen appears, press a. WAIT FOR THE SCREEN TO SAY "SAVING..." AND IMMEDIATELY TURN OFF YOUR GAME BOY. you must do this correctly, or you stand the chance of losing not only that Pokémon you want to clone, but also your data. if it is done correctly, when you turn back on the game boy, you will have the Pokémon in subject in your team, and a copy of it in the PC. you can use this to clone items, if you attach an item beforehand to the Pokémon.
Getting the 3 starters without trading
Before you choose a Pokémon from the 3 starters: Chikorita, Cyndaquil, and Totodile, save your game. Then just play without saving until you get a Poké Ball from Professor Elm's assistant. First give a pokeball to your starter. Then catch a wild pidgey or whatever. Then goe to the PC and deposit the starter, then change box. Agree to save, then reset the game right away. When you return, pick a different starter. Go to the PC and get your first starter. Take the pokeball from it and find a wild encounter. This way, you don't have to repeat route 30 over and over. When you find your wild encounter, YOU MUST CATCH IT. Return to the pokemon center, and deposit your to starters. Then turn off the console when it says : Saving... Don't turn off the power. Turn it back on and you will be back on Professor Elm's Lab and he will ask you to chose your 3rd starter. Choose the last starter. After doing this, you will have 1 starter with you and 2 starters deposited in the PC. Continue your game and Volia! You now have 3 starters (though you should just use the one that's super effective against your rival).
Unused olivine city house
This map is not able to be accessed through normal gameplay. The warp to the house is at the top left part of the map and can only be access through means of cheats. To get to the warp you must have a walk through walls cheat and a cheat that changes the warp behavior. upon walking over the warp with the codes on you will be warped to the unused house which contains two NPC's. one is a Rhydon and the other one is a female NPC. When A is pressed in front of the woman she will say "When my Pokémon got sick, the Pharmacist in Ecruteak made some medicine for me." this suggests that at one point in the game the developers were planning on putting the pharmacist in Ecruteak city.