Google and iCloud are 2 different, separate and independent things. Google can't tell you iCloud is full, because it has no idea of it, does not use it, and cannot access it in any way to know what kind of storage space there may be on it.
It's probably telling you your Google storage is what is full, not iCloud.
If your Google Storage iOS full,. then you have to deal with that on Google's website and apps, iCloud is not involved.
With that said, while you can certainly use iCloud Drive to sync photos across your iPad and Windows PC, the Android device will be the odd man out. There is no way for Android to access iCloud Photos beyond logging into the iCloud website in a web browser.
click here ➜ View your photos and videos on iCloud.com - Apple Support
On the PC, you can use iCloud for Windows to sync iCloud Photos to the Microsoft Photos App and the iCloud Photos folder in Windows explorer.
click here ➜ Use iCloud Photos on your PC - Apple Support
If you use the Google Photos App on your iPad it can sync the photos from the iPad's Photos App to Google's servers and then make them available to the Android device, but then you need storage space on Google's service for that.
https://support.google.com/photos/answer/6220402?hl=en&co=GENIE.Platform%3DiOS