The cog gallery is a very unique way to play this game. Playing with this method will eventually let you summon cog buildings, invasions, and cogs. In order to though, you’ll need to fill up the entire cog gallery.
As you defeat cogs in “Toontown” (Rewritten, Planet, Fellowship, etc.) you’ll fill up the cog gallery and along the way, you’ll be able to use summons to call in invasions, create cog buildings for the certain cog type from the cog you choose, and be able to summon a cog into the game to battle.
The certain mechanics for how many cogs exactly for certain parts of the Cog Gallery are yet to be researched. In fact, I’m one of the people who is trying to research that.