In order to migrate photos from iCloud Photo Library to Google Photos, you need to download photos from iCloud Photo Library to Mac and then back them up to Google Photos. To do so you can use Photos app and download "Google Photos Backup" app on your Mac. Google Photos works only for those photos you have locally stored on your Mac.
To do so, go to "Photos" app and turn OFF "Optimized Storage" and download the iCloud Photo Library for offline use and then run the "Google Photos Backup" app. It will check the photos which are downloaded from iCloud Photo Library by default and then click "Start Backup" button. This way you can migrate photos from iCloud Photo Library to Google Photos on Mac.
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