After finishing the story. I confirmed that the gallery fully unlocks after story completion. Also, there was only ever one choice.
How does the gallery unlock? Downloaded the game and it came with the gallery fully unlocked. Which is cool, but I like to unlock things myself. So I backed up, and deleted the content in the save file. Which cleared out the gallery. But so far, as I've progressed. Nothing is unlocking. So I'm wondering if this is the kind of game where content only unlocks when you've finished the game, or I messed something up and it should be unlocking bits as I progress.