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
mahogany town
fire
The seventh gym leader is Candice, the Ice Type leader. She is at Snowpoint City.
Mossdeep cityit is in mossdeep
Candice
On cinnibar island.