If you are thinking to upload iCloud photos to your Google Photos account, You can do that easily from your iPhone, provided you have enough free space on your phone. When your device uploads to Google Photos, it will be drawing from your iPhone's local storage, not iCloud, so you'll want the highest-quality versions of your photos on your device.
Here's how you can download iCloud photos on an iPhone for Google Photos
- In Settings, select Photos & Camera.
- Enable "Download and Keep Originals" if it's not already checked enabled. You may need to wait for this process to complete.
- Download Google Photos on your iPhone.
- Open Google Photos, and during setup, make sure Backup & sync is set to On. Now, your photos downloaded from iCloud will start uploading to Google Photos.