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. We use 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.
The 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 for transfers to Google Photos.
To start a request to transfer the photos and videos in iCloud Photos:
1. Sign in with your Apple ID at privacy.apple.com.
2. Select Transfer a copy of your data.
3. Follow the prompts to complete your request.
Your Google account’s total storage space applies to content transferred from iCloud Photos to Google Photos. Google Photos also has a limit of 20,000 photos per album. If you transfer an album with more than 20,000 photos, the additional photos are still transferred but are not added to the album.
More details in the support document linked below...