1) Order Super Mario Bros. wall clings with his birthday party supplies - but hide them and wait.
2) Secretly sneak upstairs a couple of months later when absolutely no surprise is expected to apply said wall clings while the babysitter is at the house and keeping the kids downstairs.
3) Call Gabe up saying "Gabe, something happened to your room!"
Be sure to wait with camera posed so you can catch this expression:
And this one:
When he discovers Mario on his bed and the walls a plethora of Mario's friends and enemies and other assortment of items from the Mario games.