Depends. If you got the achievement for having the full left side set for a class, you can get that class's gear from the calamity salvager any time you want. If you did NOT get the achievement though, anything you throw away is lost forever.

This is for left side items only though, right side items (accessories) cannot be reclaimed through the calamity salvager. The Garo shield cannot be reclaimed that way either, though there is a basically identical version of it that's obtainable through golden saucer.