The way you've phrased that...
"I can see all the other photos just fine with anything." Do you mean that you can see the photos in question fine when using Google Photos, let's say, or do you mean that you can see the photos that are viewable in the Gallery app just the same in another app, like Google Photos, but the questionable images are still missing or damaged in both? Please clarify what images are viewable and what photos are not, in each of the different apps.
For instance,
In the Gallery app, these photos are as described, grey boxes or whatever, but in GPhotos I'm able to see every image. OR the same photos that I'm not able to see in the Gallery app are still not visible in other apps, like GPhotos?
In the meantime, I searched this, older images on my android device show up with an overlay when attempting to view them in Samsung Gallery app but not newer ones, and got this response.
The screen overlay issue with older images in Samsung Gallery may be caused by an app interfering with image display. To resolve this, you can try disabling overlay permissions for apps that may be causing the issue or clearing the Gallery app's cache. [1, 2, 3]
It goes on to step by step you through how to identify and disable an app that might be responsible for this.
I went on to ask about the barricade icons using this search, why would some older images in Samsung Gallery have traffic barricades shown on the thumbnail, and got this response.
The appearance of traffic barricade-like images on the thumbnails of older images in the Samsung Gallery app is likely due to a bug or software issue related to how the app displays certain image types or formats. These "barricade" symbols can also be a visual indication of a file that is corrupted or cannot be properly displayed. [1, 2]
Here's why this might be happening: [1, 1]
This makes sense and so far you haven't mentioned doing anything to attempt to fix the app. My first suggestion would be to clear the cache for the app. To do this navigate to Settings>Apps>Gallery>Storage>Clear app cache. If you test the condition after this action and it still appears to have these symptoms I would recommend clearing the app data as well. In an app like Gallery, clearing data will not affect the images saved on the device. This only affects the app and should not affect files saved to the device since the Gallery app is only a viewer and not responsible for saving or keeping the actual images.
Let us know what you find.