To archive an item in Gather, simply hover over the item image, and click on the file box icon in the upper right-hand corner.
Once an item is archived, it can be found by clicking on the arrow next to the search box and then selecting "Archived Items" under the "Sort By" column.
Click anywhere outside the box to view all archived items. From here, you can unarchive by rolling over the item image and clicking "Unarchive" as shown below.
Now you will need to click on the arrow next to the search box and uncheck the "Archive" option.
The item you unarchived will now appear on the main page of your project in the area it was originally placed in.