It is possible to update Sitecore items imported from GatherContent. Just follow these steps to update an item:
Select update option
Select a node you want to update items under and click "Update" button in the ribbon.
Select items to update
The pop up dialog will show all items located under the selected node that were previously imported from GatherContent. You can select the items to update and click "Next".
View results screen
The update process is similar to the import process. It ends with a results screen where you can view how many items were updated and if there are any errors.