Actually, you can get behind both gates relatively easily these days thanks to WotG ... granted, it would be an issue if you don't HAVE WotG, but if you have it, there's a very cheap and easy way to get behind either gate. For the one gate, you just go through Garlaige in the past, before it's all blasted apart and unpassable, use the maw outside. Zone back in, and vioila, you're behind the gate.
For the other one there's a mini-quest involving a Lycopodium NPC that will allow you (once you've completed the quest) up a flight of stairs and AROUND the gate by trading a flower in the present to a glowing spot.
http://wiki.ffxiclopedia.org/wiki/Lycopodium_(NPC) after using the above method to get to the deeper sections of Garlaige.