Everything in the gallery can be unlocked.
For the Trauma section, I have the pool of blood, but not the ones before or after it.
I also do not have the Hoss 2nd CG.
So by the pictures, I am missing 3.
When I look at the completion, it says 51/53.
I am guessing one is Oswin's death (because I dont see it anywhere, but I have reached it on numerous paths/routes).
Everyone's gallery should be identical as far as the order of pictures.
As far as which ones you're missing (assuming it's the last three):
- Path C, tied to a certain choice like the one before.
- Path B, unmissable. Should have already unlocked if you'd gotten the 4th last of the gallery.
- Path A, unmissable.