Welcome to Android Central! Is this the Google Photos app, or some other stock gallery app from Samsung?
Are all of the photos on Internal Storage, or on an SD card? If the latter, I'd Unmount the card (in Settings>Storage), remove it, and insert it into your computer, then back up all of the photos to the hard drive. SD cards are not that reliable, so don't use them as the sole storage for important data. You can also see if the computer has problems displaying the images.
Another thing you can try is to Clear Data from the Photos app (regardless of which one you have). This won't delete your pictures -- it will just clear the gallery's database, and the system will rebuild it as soon as you reopen the app.
09-11-2018 11:10 PM