Go to saffron city and go through the silph co building and beat Giovanni again then go to the gym to get your 6th badge.
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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
To get the fifth badge in Pokémon FireRed and LeafGreen version you have to beat Koga. He uses poison type Pokémon.
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