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You need the Silph Scope, which you get after defeating Giovanni in the Rocket Game Corner basement.
Go to Viridian city and beat Giovanni,the gym leader
Save Silph Co from Team Rocket by defeating Giovanni for the 2nd time, after that you will get the Master Ball! Then you will have access to Sabrina's Gym.
you mainly unlock duelists by defeating a person in duel worlds 5 times but you can also unlock duelists by defeating the gate to the next world or defeating people in CPU mode hope that helps!
In Sliph co., just before the fight with Giovanni, and after the fight with the rival, there is a man that gives it to you