How do you unlock the 8th Gym Leader Door in Pokemon Ruby Version?

You don't unlock the 8th Gym Leader door in Pokemon Ruby.

Here's why:

* Pokemon Ruby has only 7 Gym Leaders. You fight Norman (Normal), Flannery (Fire), Winona (Flying), Tate & Liza (Psychic), Juan (Water), Brawly (Fighting), and Wallace (Water).

* The Elite Four and Champion are your final challenges. After defeating all 7 Gym Leaders, you will face the Elite Four and then the Champion, Steven Stone.

There might be some confusion, as you might be thinking about a different game or a different location within the game. Could you clarify what you are referring to?