Apple has enabled a service designed to make it easier for iCloud users to transfer photos and videos to Google Photos.
As detailed in Apple's support document, you can go to Apple's privacy web page to log in and use the “Transfer a copy of your data” option. If you select it and follow all the steps, Apple will transfer your photos and videos from iCloud to Google Photos.
Transferring photos and videos from iCloud Photos does not delete the content you have stored in your Apple account, but instead creates a copy of the content in Google Photos.
The transfer process takes three to seven days, and during that time Apple will verify that the request is legitimate, and make the transfer. Apple requires that you have two-factor authentication turned on and that the space in Google Photos is equal to or greater than the space required for the document transfer you are about to do.
There are some formats used by Apple systems which are not supported by Google Photos, such as Smart Album, Live Photos, RAW files, etc. but the usual formats (.jpg, .png, .webp, .gif, .mpg, .mod, .mmv, .tod, .wmv, .asf, .avi, .divx, .mov, .m4v, .3gp, .3g2, .mp4, .m2t, .m2ts, .mts, .mkv, etc.) are supported.
Only the most recent change of a photo (rather than the original) will be transferred, and the duplicates will appear as a single image. Whenever possible, the albums with their pictures will be kept inside, but the videos will be transferred autonomously.
The iCloud photo transfer service to Google Photos is currently available to users in the European Union, Australia, Canada, Iceland, Liechtenstein, New Zealand, Norway, Switzerland, the United Kingdom and the United States.