Pictures are not deleted from "Photos" iphone app when deleted from "Google Photos"
When deleting on my iPhone, it asks if I will "Allow "GooglePhotos" to delete this photo? This photo will be deleted from iCloud Photo Library on all your devices." I can choose not to allow it and it will only be deleted on my Google Photos app. 

I evidently have iCloud Photo Library turned on, and backup and sync on the Google Photos app turned on as well.

Also, check on Settings > Privacy > Photos to see if Google Photos is allowed to access your pictures.