I haven't done it legitimately so I may have missed something, but I used Cheat Engine to set my treasures to 12 and open the chest at the beginning in order to see what happens. There's a rainbow-y version of the treasure inside, which increases your treasure count to 13 and teleports you to the final room.
If you've beaten the game without collecting everything and it didn't unlock the gallery, then it unlocks that as well. (I've only beaten the game on the save where I cheated in the treasures) It's a small environment meant to let you experiment with the different enemy interactions. The only one that I think you'd be likely to miss in normal gameplay would be the one where a mothgirl that's carrying you gets grabbed by vines, and only because you'd usually fall a long distance in any room where that's possible.
(And if you're just missing one treasure, the one I found that I think would be the easiest to miss would be the one you get by backtracking a couple screens from the first place you can become a mothgirl and flying beyond the top of the screen once outside the mountain)