Google Photos is one of the most used photo backup services as it is free to use. But your Google drive do get used if you are uploading very high resolutions photos. Visit https://one.google.com/storage to know which google app is taking most space. Once you do that and if you see Google Photos in it you can either delete them or better you can downgrade it and still keep it by visiting https://photos.google.com/unsupportedvideos. Here u can see the unsupported videos (by Google Photos) and you can compress it and gain some space back.