A new Apple support document tells you how to transfer a copy of iCloud Photos collection to another service.
You can request to transfer a copy of photos and videos you store in iCloud Photos to Google Photos. Transferring photos and videos from iCloud Photos doesn’t remove or alter the content you store with Apple, but sends a copy of your content to the other service.
The transfer process takes between three and seven days. Apple says it uses this time to verify that the request was made by you, and to make the transfer.
Some data and formats available in iCloud Photos—such as Smart Albums, Live Photos, or some RAW files—may not be available when you transfer your content to another service.
This service is initially available to customers in Australia, Canada, the European Union, Iceland, Liechtenstein, New Zealand, Norway, Switzerland, the United Kingdom, and the United States