A man with a plan and a butterfly costume and another man with a butterfly costume.
But most importantly, a plan.
He knew that, sooner or later, the Joker would get cured of his zombism. Maybe he and Roy would even do it themselves. Either way, Joker would be mad the next time he saw them (making the very safe assumption that they would not have Batman in their custody when this happened) so they wanted to give him a present to soften the blow. Gary didn't know much about Shakespeare, but he did learn about this item on a guided tour of the museum in his downtime. It was perfect for his purposes.
This plan didn't include much about breaking into a museum, though. Roy handled that part on the spot, picking open the lock on the skylight above the main gallery. They drifted carefully down to the floor and looked around. Suits of armor from various eras and regions lined the walls, and the floor was dominated by a mannequin on a mannequin horse, both covered in shiny plate mail.
"Woah," marvelled Gary. "What I wouldn't give to have one of these to wear at a LARP."
"Shut up ..." warned Roy. "I think I heard something."