About Gogo, life will be a lot easier if you have Mog in your party and use the Moogle Charm. You won't get into any fights, and I always found the inside of the Zone Eater to be frustratingly hard. You can get that by searching the wall where Mog stood when you find him in Narshe.

And in case you don't know already, you can't have Odin and Raiden. You find Odin by going up to his statue, and then down to the next level to level that Magicite up to Raiden.