maybe your problem is you think it is the first. just move on until you have beaten all other 7 gyms and then you can get in.
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It is in Viridian City. After you have beten the 7th gym leader it is open
You have to beat the other 7 gyms then when u leave 7 gym bill will come out and you will go somewhere blah blah blah do all the stuff go to viridian city there open then the gym uses ground type there
At the end of the game, 8th gym, Giovani, head of Team Rocket.
Go to Viridian city and beat Giovanni,the gym leader
Defeat every other gym. The Viridian City gym is actually your last gym, not your first.