The last gym is the Earth gym in Viridian city. The one that was closed at the start of the game.
Viridian city.
Go to the Pokemon League, West of Virdian City
if you have Groudon and it has fissure use him..
you should have a lvl 50 water type
no
In his Gym in Cinnabar.
it is in saffron
beat the gym leader
the thunder gym in in vermillion city
In the cinnibar island gym.
You have to beat the 2nd gym.
there is no gym leader in lavender town