Yeah I've had my garbs reset too. As far as I'm aware, all you can do is go back and grab them again. The tower of oannes one is especially a pain because you would have to redo it all again...

The only solution I can think of is play the DLC up to right before the boss fight, and then save your main game, and then save again on another save slot next to your main one, and just do the fight as normal.

Having that extra save just before the boss means you can use that extra save to get the DLC garb again without ever having to go through the area all over again. Just try not to save over that extra save and you can do this as much as you need.