tomatoSoup It sounds like the gallery app doesn't have that option. It's an AOSP app and doesn't get much attention from the Android developers.
You might consider using a different gallery app instead, or you can use another app to edit your photos. One that I've used a few times and has been suggested by other members of the community is this one: https://github.com/T8RIN/ImageToolbox. If I remember correctly, it always saves a new copy of photos after editing.