I have only done this in Pokemon Yellow, but this should be the same in Fire Red and Leaf Green. On Cinnibar Island there is a building that suffered a fire and is now in ruins. Inside that building is a total maze of rubble where one can find a scientist's journal revealing that Mew was actually under research there and had "given birth" to Mewtwo. In this building will also be a key that opens the gym doors. In Crystal, Gold, or Silver the gym has been relocated to the Seafoam Islands due to a volcanic eruption. These islands are in the sea to the right of Cinnibar Island. In a cave on the only one of the islands you can go to you will find the leader, Blake. Hope this was helpful.
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You take it to the Pokemon Lab on Cinnabar Island, a scientist there will make it into a Aerodactyl.
Its in burnt tower and if u wanna make it evolve into a magmortar u need the magmarizer. PS: the magmarizer is in CINNABAR ISLAND IN KANTO
You need to go the other way go to pallet town and surf south you will reach cinnibar island soon.
It depends on what you've already done but make sure you beat Sabrina, the Gym Leader there first. If you went to Fuchsia City already and beat Koga, the Gym Lleader there and you already got the HMs of Surf from within the Safari Zone and the HM of Strength by retrieving the Warden's teeth and giving them to him so you get the HM of Strength and you can now go to Cinnabar Island. You can either go get there by Surfing on the water south of Fuchsia City and head west or you could Fly to Pallet Town and Surf all the way south until you get to Cinnabar Island. Next you go into the Pokémon Mansion to find the key to the Cinnabar Gym.
After beating Brock in Pokémon HeartGold and SoulSilver, you next head down to Pallet Town and then Surf on the water route down to Cinnabar Island. Once you get to the Island, Surf on the tiny path of water down below the Island and head east and you'll make your way to a cavern that is part of Seafoam Islands. Head into this cavern and you'll encounter Blaine in his new Gym. After beating him, head back to Cinnabar Island and talk to Blue there and he'll return to his Gym provided you have 7 of the Kanto Gym Badges. Once you get the 7 Kanto Gym Badges head to Viridian City and go into the Gym there and challenge Blue to a Gym Battle and defeat him.