First you have to get HM05 (surf) from the safari zone. It is the reward for finding the 'end' of the safari zone. then fly to pallet town, and go to the southern most part in the town and surf on the small bit of water sticking out. Continue to surf down and you will eventually find the island where the 7th gym is. To get in you need to go to the large building to your immediate left (if you see a carpet going up, it is the right one, if not it probably the one above it) you need to get to the end of this area by using the various switches located in the statues. When you get there you should find a 'secret key' this is used to unlock the gym door.
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Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
you have to find it
In the Pokemon forest
On the way to Snowpoint City. Behind a cottage on the way, you will find the HM Rock Climb. You can use that once you beat the seventh gym leader Candice. She uses Ice-Type Pokemon so have some good Fire-Types in your party!
easy if you want to challage the Pokemon leader you have to go to the light house and go to the top and find the gym leader then you challage the leader get the HM then you go to the Pokemon league