Well, duh, it's the gym in Viridian city which uses ground-type pokemon.
it is in the south of Fuchsia City. the gym leader is Koga.
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.
you go to the 4th gym leader.her name is erika.her strongest Pokemon is a level 29 viloplume.