Where is the sixth gym located in emerald?

Answer 1

The Fourth Gym is located in Foretree City. To reach it you need to get surf from Wally's father in an event after defeating your father (Petalburg City Gym Leader). Then you will need to surf your way there from Mauville City. To do this Surf to the Right along Route 118 and then walk north along Route 119. You will have to walk most of this route as it is covered in tall grass. (Take this opportunity to catch a Tropius [9% appearance rate in the tall grass])

Once in Fortree, You cannot immediately enter the gym because something invisible is blocking the way. Exit Fortree city heading right from the right most side across from the pokecenter and then go south, you'll find a bridge and in the middle will be Steven. He will give you the Devon scope and trigger a combat with a Kecleon to demonstrate the scope so be ready if you plan to catch it. (Stock a bunch of Ultra balls from the shop in Fortree) If you don't catch this one you cannot catch the one in front of Fortree's Gym as it flees without combat. If you miss out on the first one you can find more Kecleon (Rare) in locations detailed on Bulbpedia:

Example: 1% appearance rate in grass on route 119

http://bulbapedia.bulbagarden.net/wiki/Kecleon_%28Pok%C3%A9mon%29#Game_locations