Pattern Bush

Pattern Bush is a part of Green Path. It's a sort of forest with a ton of wild Pokemon and trainers. You want to go to the southwest corner of Pattern Bush to get out. After you do, surf north. You'll be going through a group of islands called Outcast Island, and soon you'll reach a cave with a Rocket member standing outside. This is the Altering Cave, and the only Pokemon inside are Zubat. However, the Pokemon changes if you use Mystery Gift. There isn't much to do here except for catching the Pokemon in the cave. Now either fly back or walk back to Six Island.