Google Photos Sync is a command-line utility which downloads your Google Photos to the local file system. It will backup all the photos the user uploaded to Google Photos, but also the album information and additional Google Photos 'Creations' (animations, panoramas, movies, effects and collages).
In order to work, gphotos-sync first needs a valid client id linked to a project authorized to use the 'Photos Library API'. It is not provided in the distribution. Each client id is given a (large) limited number of free API calls to Google Services. If this distribution shared the client id, all users would share this resource limit. This is a little fiddly but only needs to be done once.