1) IN-GAME LOCATION: the gallery will be an actual stage in the game where Maya can move freely and even teleport to. This gallery stage is aptly located inside the castle and is accessible via the main hub there. You need the sliding tackle to get in there, so in essence the gallery mode is "locked" until you defeat the hill boss to acquire that skill, which can be done relatively early. Once you enter the gallery for the 1st time, this will also unlock the gallery option in the title menu of the game, which if selected will simply load directly into the gallery level. The fun thing about having an actual stage is that I can add a lot of interesting things like the enemy statues that you can see above in the 2nd image. Maya can read the description of each statue to get a bit of lore and even some secrets.
2) ANIMATION LIST: when Maya is inside the gallery stage, you can pause the game and there will be a new option in the pause menu called "Animations". As you've guessed, it will contain a list of all enemies in the game and their respective animations. This will allow players to peruse any of their favorite animations and play them at will.
3) SPAWN ENEMIES: in that very same list, players will have the option of instantly spawning their preferred enemies in the game world, allowing players to setup specific mob fights for a challenge battle, or assemble their favorite enemies grouped in the same space to gang-up on Maya.
4) ANIMATION CONTROL: this will be a small window pop-up that can be enabled whenever Maya is grabbed in the game world, or when players are viewing animation in the list. That small window will have a set of controls such as play, stop, rewind, go forward, etc, essentially giving full control of each animation scene. This allow players to loop a certain phase of their favorite animations for however long they wish, and even set a custom speed.
I missed this being posted, but here's what you can look forward to in the gallery