If I delete Google photo app backup will it delete my iCloud

Yes, if you use the free up space option on Google Photos app, it will delete the pictures from your iOS Photos app/iCloud Photo Library. For that reason, I have never used that option. You can backup your iOS Photos to an external hard disk first and then use free up space. In the end, I bought a SanDisk ixpand iPhone usb drive, which easily backups up iOS Photos but it's not cheap so it might not be the right option for you. I needed it because I was filming a lot abroad and I didn't have a computer I could backup the video files to, to free up storage on the phone.

Sorry for the late reply. I'm pleased it helped you. I got the 32GB one I think (I double-check when I get home, and amend this message if I'm wrong) - for two reasons, 1. these things are fairly expensive 2. these things are, like every USB, easy to lose. So I reasoned that if I get the 32GB, I can only lose that much data and that much money; both of which scenarios would be worse with the 64GB. I must say, it is one of the best things that I ever bought, I really love that little thing. Never again will I have to worry when filming or taking photos in the field, as it were [especially abroad], that I'll run out of storage space and have to start making painful decisions about what to trash and what to keep.