Option 1...
Try restarting the device? Sometimes that is all you need to do.
Option 2...
You may want to then back up the images to a cloud account or a computer (via USB cord) and then clear out the data and cache (settings, apps, all) of the gallery, then restart the phone.
Option 3...
Should that not fix it, next I would try uninstalling updates made to the gallery (same location as the data and cache) then again restart.
Option 4...
Still problems? As a last resort you may want to try a "Hard Reset".
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to...