it is in the south of Fuchsia City. the gym leader is Koga.
Well, duh, it's the gym in Viridian city which uses ground-type pokemon.
You find badge 5 in fushia city and to get there you need to go down cycling road. Koga is a poison gym leader. He hs a weezing and i think a hypno.
The eighth badge in Viridian City. The gym leader is Giovanni. In order to enter the gym you have to have all seven badges, and have beaten Giovanni at Silph Co. and Rocket Game Corner.
Defeat team rocket in silph co then you can go into the saffron city gym then beat the leader to get the 6th badge.