This cheat works in Pokemon Blue, Red and Yellow, the only thing you need is HM 03 and HM 02 and Pokemon which you can teach this technics.
Go to the Safari Zone and when your in the safari zone go back to the entrance.
Say no on the Question "Leaving Early" and you'll go back to the safari zone. go back to the entrance and say no again.
Save the game and turn off, turn on again and go back to the entrance, walk to the dude you paid and say no, get out of the building. fly to Cinnabar Island and go surf right coast (the same coast is used with the Missingknow cheat).
Keep on surfing up and down stay on the vertical coast till there will appear: "PA: Ding Dong" you'll get back to the entrance from the Safari zone; "Did you get a good haul, Come again!" leave the building.
And then you are in Chaos Town (I call it this way, you'll know why when you're there).
It is said that here is a ninth gym... the price is the Mist Badge, wich make you're Pokemon incredibly strong. I still didn't find but keep on searching.
The Gym leader is towards my sources Missingknow Himself, and he has got just one pokemon called Mewthree wich is the Spirit of a Mew captured in the steel armor made by Giovanni for Mewtwo, you can see the armor in the first movie of Pokemon.
WARNING: Don't save in Chaos Town, you'll can get stuck and then you'll need to start over new: when you get stuck you can fly away...
When someone finds the 9th gym please post here.