After you find the Secret Key in the Pokémon Mansion on Cinnabar Island, you can enter the Cinnabar Island Gym.
Blaine's Quiz Show
When you're ready to challenge the gym, talk to Blaine and say Yes. You have to collect five Blaine dolls to challenge Blaine himself. You can get a Blaine doll by answering a question correctly. If you answer incorrectly, you will battle a trainer, then get a Blaine Doll for defeating that trainer.
You might want to answer incorrectly on purpose to get more EXP and Pokédollars, but it is up to you.
Which of these is the Spitfire Pokémon?
If you answer incorrectly, you battle Super Nerd Erik, who has a level 45 Charmeleon. You get 1800 Pokédollars for winning.
How many badges certified by the Pokémon League are there?
If you answer incorrectly, you battle Burglar Quinn, who has a level 44 Ninetales. You get 3520 Pokédollars for winning.
A Steel-type move used on a Fire-type pokémon will be...?
If you answer incorrectly, you battle Super Nerd Derek, who has a level 45 Magmar. You get 1800 Pokédollars for winning.
True or false! TM28 contains the move Tombstony.
If you answer incorrectly, you battle Burglar Dusty, who has a level 44 Growlithe and a level 44 Rapidash. You get 3520 Pokédollars for winning.
Who are you about to battle?
Gym Leader Blaine
Gym Leader Blaine has a level 47 Magmar, a level 48 Arcanine, a level 47 Rapidash, and a level 47 Ninetales. You get 7680 Pokédollars for winning.
After the battle, you get a Volcano Badge, which increases the level at which traded pokémon obey you. You also get TM46 Fire Blast.
When you go outside, there will be a cutscene. Your rival says the final gym is in Viridian City.
When you go to the Viridian City Gym, you find that it is still locked. During a cutscene, say that you should talk to Professor Oak about the locked gym. You will automatically go to his lab in Pallet Town. While there, Blue will give you a Key Stone, as well as a Venusaurite, a Charizardite X, a Charizardite Y, and a Blastoisinite. You will automatically have the option to Mega Evolve these pokémon (Venusaur, Charizard, and Blastoise) during battle. You can only do one Mega Evolution during a battle, and the Mega Evolved pokémon will return to normal afterward.
When you leave the lab, there will be a cutscene, and you find out that the Viridian City Gym Leader has returned. If you go into the Viridian City Gym, you find out that the trainers there use Ground-type pokémon.
You can go there next, or, if you haven't already, you can check out the Power Plant by surfing in Route 10, or you can stop by the Seafoam Islands by going south from Fuchsia City or by going east from Cinnabar Island.