The Gallery is the third place you will unlock as you collect keys from clearing levels. There are 24 levels here in all. Once it is open go there to complete its stages. This will unlock once you clear them all in the Gallery. The Gallery hosts some interesting levels that involve shrinking and super sizing Shuggy in order to collect green gems. Some notables:
One level called "The Rope Balance" you must grab the rope from the purple rope box with RT then wrap the rope around the small box in front of you counter clockwise. Get on top of the box then jump and press Down + RT. With balance, you'll avoid the spikes and get to the top.Black Ghost Boss
The "Gallery Ghoul" stage opens once you obtain enough keys. It will be the door to the very right-middle area. There you must defeat a black ghost by hitting a combination of certain switches. Switch between Shuggies by pressing RT and avoid the portraits from hitting any of your Shuggy clones. Keep pressing and holding different buttons until you zap it with a laser. Continue until it drops green gems for you to collect and finish the stage. Below is a video run of the Gallery if you get stuck.