Open the app manager and go to Gallery. Clear the cache and see if that works. You could also try a force stop of the Gallery to see if that fixes it. If not, wipe the cache partition, instructions below. The cache partition wipe will take care of a lot of weird, unexplainable issues, at least it has for me on my Android devices over the last 10+ years.
Note 8 wipe cache partition:
Back up data on the internal memory.
Turn off the device.
Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
When the green Android logo displays, release all keys ('Installing system update' will show for about 30 - 60 seconds before showing the Android system recovery menu options).
Press the Volume down key several times to highlight 'wipe cache partition'.
Press Power button to select.
Press the Volume down key until 'yes' is highlighted and press the Power button.
When the wipe cache partition is complete, “Reboot system now” is highlighted.
Press the Power key to restart the device.