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Gym is at the top of a small mountain. A gate at the base will bar you from entering. You have to use the Pokemon in your party with the highest ATK stat to push it open. You just mash A or something to open it. After that, you can proceed up the mountain.
You fight a few trainers on the way, and then meet a Sawk/Throh duo with no trainer at the top. After you defeat them, the gym leader will come to greet you. Cue opening line and music start. You acquire the Force Badge, which enables you to use Strength, and the TM for Bulk Up after winning the battle.
