To get Gogo into your party, you'll want to head to the top right portion of the map where the little triangle island is, which is also where you could fight the Intangir in the world of balance. Gogo's level will be equal to the average level of your party when he joins, so keep that in mind for when you decide to get him. He also does not have to be taught any magic and can use any skill in the game. Anyway, heading to the triangle island, stick around until you find a zone eater and just stay put until it engulfs your entire party.
You'll find yourself inside a cave with stairs going up and down. The stairs up lead out of the zone eater and into the world map, so take the stairs down. There will be some stairs to your bottom right that lead to a lower area, and a bunch of walkways to the left. You'll almost certainly want the sprint shoes for this part. If you're next to any of the green men as they walk back and forth they'll push you off and you'll land in a lower area. Let them do this once and you'll get an ether and a red jacket, and hit the switch to climb back up the stairway you saw in the previous room.
Make your way across the walkways with the green men. For the first two chests at the bottom make sure you approach them from both sides to get the Genji Armor and the magical brush. Whenever you find yourself on a "safe" ledge just wait until the green guys move into a far enough away position for you to pass. Make sure you get the chest in the bottom left area for a fake moustache relic which lets Relm control instead of sketch monsters.
Make sure you save in the next room because you'll be in a very tough area where it's easy to get killed instantly. The ceiling will periodically fall and crush you if you're under it. Make sure you have the sprint shoes and start walking and stand right below the first chest you see before the ceiling falls. As soon as the ceiling begins to rise start walking and stop right below the next chest that you see. The next spot you have to reach is hard, it's the third square down in a middle section of the ground, look for where the hole in the ceiling appears to be and walk three steps from the top of that column and you should be ok. The next hole in the ceiling is in the top right corner of the part of the platform with the chest, make sure you don't try to stand in front of the chest so you don't get crushed. If you're in the top right portion you should be safe. After that, just make a mad dash for the exit and make sure to grab the last chest too.
You'll pass through a room and find yourself in a room with a bunch of platforms you can jump between and chests you can bounce on. Walk around the path and jump onto the platform with the switch to open up some more paths. Head back to the start and jump over to the bottom left and up to reach a chest with a thunder shield. Then head back around to where you jumped onto the platform with the switch and you can reach a platform which has a door. The door will take you to the master of mimicry, Gogo himself. You can select status on him from the menu screen to equip him with the abilities of any character, and if you equip him with the merit award which you can win in the Coliseum you can give him any weapons or armor and make him truly a force to be reckoned with. He never needs to be taught any magic and will automatically know the spells of everyone in the party if you give him the magic command. Use the warp spell or a warp stone to exit if you like or just make your way out of the zone eater in reverse.