Cinnibar Island.
The 8 badges for Pokemon Red are: Boulder Badge, (Brock) Cascade Badge, (Misty) Thunder Badge, (Lt. Surge) Rainbow Badge, (Erika) Marsh Badge, (Sabrina) Soul Badge, (Koga) Volcano Badge, (Blaine) and Earth Badge. (Giovanni)
you sneak into it and Get in there go get a rode and do it
You need the rainbow badge which you get a celadon city Hm cut is needed.
Blaine, the gym leader of cinnabar island, gives it to you after whooping his butt.
beat the 7th badge, and you will be taken to it
beat the gym
You can obtain the 8th gym badge (the earth badge) in fire red from Giovanni in Viridian City.
It is your rival from Pokemon fire red and leaf green versions.
to get the eight badge go to viridian city after you have 7 badges and enter the gym
you have to defeat the fuchsia gym leader.
You must go to the Pokemon Gym. Beat the gym leader and he awards you with the 8th gym badge
In Cinnabar Island, south of Pallet Town.