If you delete from Google Photos on the web from your desktop, the iPhone client will prompt you to allow deletion of those same photos from your iPhone’s Photo

If you delete from Google Photos on the web from your desktop, the iPhone client will prompt you to allow deletion of those same photos from your iPhone’s Photos app (and hence iCloud, I think). So, just do your deletion on the Google Photos web client on desktop. The culled photos will be culled from Apple Photos as well.