Question

How do you get to the 8th gym leader in alpha sapphire?

Answer 1

You have to get to Sootopolis City where Steven and Wallace will be waiting for you. They will take you outside Origin Cave, where Groudon lies. Once you catch / defeat Groudon, you can make your way to the Sootopolis Gym in the center of the city and it should be open.

Answer 2

Beat/catch Groudon

The Sootopolis gym door should be open after that little Groudon event. If you kill it, beat the elite 4 and it should reappear.
Hope I helped!
-It's a secret!