There are several threads about sorting photos but none that I have found solve the problem. I don't believe there is an easy way to do it because it appears to use metadata dates.
After much work, I found the way to sort photos in the Gallery. I used a metadata editor called PhotoMe. It is free on line.
When you open a photo, you should find a metadata tag ID 132 called Image/File Change date and time. You can change this using the application PhotoMe. Adjust the dates so that the first file has the latest date.
Note: I had one photo that did not have this tag. I had to export the EXIF data from a photo that did have this tag and then import it into the one that did not. Then change it.