I'm assuming you mean how to get past the invisible walls. To get to Koga, follow the perimeter of the gym to the right, until you reach the northwestern corner of the gym. Head downwards until you're right about in between the juggler and the Tamer. there will be a breach in the wall, and from there, just find gaps until you reach Koga.
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i'm pretty sure you get it from Koga when you beat him(Koga is the 6th gym leader)
you need to have all ground type Pokemon to beat koga i would say a a nidoking of nidoquenn you need to have all ground type Pokemon to beat koga i would say a a nidoking of nidoquenn
After defeating Koga the fuchsia gym leader he gives you the TM Toxic
he is the fushia gym leader, u need to get passed the snorlax to get to fushia city which requires the pokeflute
The fifth Gym can be found in Fuchsia City. The leader is Koga, who specialises with Poison-type Pokémon.