I was able to recreate your problem and got the same error message and I think I have an explanation but no real solution.
The camera app has no access to device storage, but any app can save certain kinds of files to corresponding folders, i.e. a camera app can save photos to the photos folder but can only access files it creates. So, the GOS camera wouldn't be able to open Google Camera photos, for example. If you use GOS Camera app to take photos you should be able to see the recent photos there. If you want to see photos from both apps, it's better to just use the gallery app since it should have access to file storage.
If you've already been using GOS Camera and it's throwing that error message then I have no explanation.