You can use the Google+ Auto Backup feature to keep your iPhoto Library synced with Google Photos. Now iPhoto is replaces by Photos app, so what if you want to Auto Backup Google Photos with new Apple Photos app?
Google Photos has a Auto Backup feature on Android and iOS device. But, there are some compatibility issues with the new Photos app for Mac. As a temporary, to solve this issue, you can create a symlink to your Photos library's "Masters" folder which Auto Backup allow you to select.
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