Export ~/.local/bin in PATH if you haven't already (that's where the executables for packages installed using pip3 are stored):
There are still API limitations, but this way you'll be the only one using it. This tool uses the Google Photos API, and requires you to create a new Google Cloud project, enable the Photos API for your project, and setup OAuth 2.0 for it, so you can use your own API key, to avoid running into API limitations. See how to install gphotos-sync on Windows.
The first full sync creates 2 directories: It cannot upload photos to Google Photos. This application is only for Google Photos download / backup purposes. Gphotos-sync can backup all the photos an user has uploaded to Google Photos, as well as additional photos created automatically using Google Photos, like animations, panoramas, movies, enhanced photos, or collages. This tool is specially useful after Google's announcement that Google Photos will stop syncing to Drive starting July 10, 2019, which means that some tools for backing up Google Photos to your computer will stop working ( affecting Insync for instance, a commercial Google Drive client that runs on Linux, macOS and Windows). Use it periodically to grab all newly added photos, keeping a complete Google Photos backup on a server or desktop. Gphotos-sync is a command line tool for backing up Google Photos (including separate albums) using the Google Photos Library API, for Linux, macOS and Windows.