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
it is in saffron
In his Gym in Cinnabar.
the thunder gym in in vermillion city
You have to beat the 2nd gym.
In the cinnibar island gym.
beat the gym leader
there is no gym leader in lavender town