Items in Gather can only be unarchived from within a project. You cannot unarchive items from the Library.
To unarchive an item, open the project while in Grid View and roll over the search field in the upper left corner. ***Please note: The search bar is not available while in Spreadsheet View. Click on the sort icon and select "Archived Items" under "Sort By".
This will then bring up all the archived items in the project. From here, you can unarchive a single item by clicking "Unarchive" on the item.
You can also unarchive multiple items at once by selecting the items and clicking "Unarchive" from the "Action" dropdown.
When you're done archiving, go back to the sort menu and uncheck "Archived items". This will take you back to the main view of the project.