- Starting from below the heart area, turn into a moth.
- Traverse the narrow dripping tunnel to the right by timing your movements.
- Go into the area with an ant and a meat block (by flying up the left passage, dodging the two falling droplets, lay an egg to get rid of the block, then circle back to this location from the start.
- This time, avoid the ant, flying up to the next level above that one through the gap you just opened.
- Turn into a human with the ant on your right hand side, so that it can knock you across the room through the two waterfalls.
- Be prepared to dodge the ants below you by jumping! The second one will reset you back to step 4 if it hits you.
- Turn into a moth, then fly up to the chamber with an ant and a meat block, and lay an egg.
- Return to the area below the heart, and fly to the upper right corner of the room once in moth form, flying through the waterfall and returning to human form. Bump the ant off of the raised platform (this is easiest from below), and let it knock you back across the room in human form.
- Destroy the vines, get eggnant, and dodge the last water droplet. (Remember that you can hold down to speed up your descent.)
- Destroy the large meat block in the center of the room.
- Return to human form, and hit the heart with your sword three times to collect it.
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!