Welcome to the forums. It may be possible that some of those locations are syncing to other locations. For instance, Gallery may be syncing to Google Photos.
Here is what I would suggest: If you have access to a PC, you can do a 'Takeout' from Google Photos (Search Google Photos Takeout; it should give you a direct link to your own Google Photos account). This will download all your photos on Google Photos to a series of .zip files (there is a predetermined file size, once a collection of files hits that size, a new .zip folder is created). Once this is done, you can delete all photos on Google. See what is left in other places, and make copies of the ones that are remaining. Delete those. Now, you have a full copy of your photos, where you can decide where you want them. Before placing them anywhere, I would see if there is syncing going on between apps. Don't forget to check where your camera stores them.
I would honestly suggest keeping them on Google Photos. That way you can access them from any device that can access the internet, and if your phone gets damaged, they won't be lost due to damage.