You should be able to get into the gym. Unless there was some glitch, I'd try again. You might also have to wrong key. Also blaine might be on a boat check there.
You need to acquire the Secret Key that can be found in the basement of the Pokémon Mansion and then you can use the Secret Key to enter into Blaine's Gym.
bottom floor of Pokemon Mansion
Although i have not played the new version of gold and silver, he was on cinibar island on the old games. the real cinibar island though, not blaines secret gym
Blaine's Gym is in Mahogany Town.
You explore the mansion until you come to a room with two houseplants. There'll be a pokeball there. Pick it up. It's the key.
Go to seafoam islands, and climb the latter.
The Secret Key that you get in the Pokemon Mansion is used to enter the Cinnabar Gym.
up the stairs in seafoam island
You must enter the house above the lab then you look for the secret key. that unlocks the gym You must enter the house above the lab then you look for the secret key. that unlocks the gym
Head over to Seafoam islands where Articuno is located.
Go to Blaines gym underneath there is a series of floors which lead to Articuno
Blaines Gym now resides in the sea foam islands when you enter the islands take the ladder and you'll be in his gym.