If you use use images from Google Photos or Flickr, you don't need to worry about manual synchronization. Depended on the configured cache period they are updated automatically. That does not work for local files.
For local files you can use the Joomla! command line interface (aka CLI). You can create a CRON job to sync the database. No need to trigger the synchronization from the back end manually. Just run
cli/eventgallery-sync.php from the command line. Please keep in mind that new folders are marked as unpublished. If you choose a folder name which contains the date in the format
2015-12-31, it will be automatically used as the events data and removed from the display name. In addition underscores will be transformed into space characters to fill the display name.
If you want to start the sync manually you need to use thebutton. Remove the caches for Google Photos and Flickr images to force a reload. Use the button to start a synchronization of local and Amazon S3 files and folders.
Please do also have a look at the documentation for the Sync Database process.