Have you gotten the 12 treasures yet? if you have does it award anything different from beating the game with them all or am i just wasting time?
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)
The biggest factor that's probably setting you back is that CE defaults to looking for 4 byte values, while GameMaker games tend to use doubles. (For GameMaker games in general, it's usually helpful to go to edit -> settings -> scan settings -> and tick the "all type contains..." double + 8 bytes so you can scan for both at the same time with "all," but just searching for a double works here)
Starting the game, scanning for a double with value 0, grabbing the first treasure, scanning for any that have updated to value 1, and grabbing the second treasure + scanning for 2 should narrow it down enough that you can try changing values and backtracking to the chest. You can grab the third treasure while backtracking if you're careful about avoiding the water droplets/vines to be totally sure, too!