I have a macOS 10.15.7 can I move Google photos to iCloud?

If you want to put "Google Photos" on iCloud, as opposed to photo files contained in Google Photos, that's not going to work.

iCloud is an interactive file synchronization service. It's designed to house files, not to run apps from or to contain other "systems" or services.

If you have space available, you can put any file you wish in iCloud.

Here is a very straight forward article on iCloud's use: Access and manage your files in iCloud Drive - Apple Support

I'm 76 and iCloud is not beyond us "old guys".

The simplest way is to set up synchronisation between Google Photos and the Apple Photos. I have this set up on my system.

https://support.google.com/photos/answer/9845656?hl=en-GB