There are two ways to reach Cinnibar Island where the seventh gym is present: You can Surf south from Pallet Town until you reach the island or Surf southwest from Fuchsia City.
Keep in mind that using the Fuchsia City method you will need to go through Seafoam Islands to reach Cinnibar Island. Therefore the quickest method is from Pallet Town.
use a Pokemon to get to use "surf" then get to cinnibar island and there you go.
the seventh gym is at sekka city
The 7th gym leader is Blaine. He the gym leader of Cinnabar Island gym. He uses Fire type Pokemon and he will give you the TM that contains Fire Blast when you defeat him.
You need to surf South from Pallet town to Cinnabar Island after you have the first 6 badges.
In the mahogany gym
In gym number seven
pryce the ice type gym leader
fire
mahogany town
The seventh gym leader is Candice, the Ice Type leader. She is at Snowpoint City.
Mossdeep cityit is in mossdeep
Candice