I was just about to post about this,but i have just done it
I started on the railing at the back,and jumped on the vent/window frame and then went from up there,and then round the front on top of the shelter part and finally jumped up to the ledge where the fencing was broke and then into the room...one of the safes were bright white...not sure if that was a bug or not,but i still got money from there
that final place opened up and i got my achievement
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I've had many questions about 'The Amazing World of Gumball' gamerpic,it was part of a competition on the XBL UK dashboard a couple of years ago(download to enter kinda thing),and they don't seem to be available for normal download at all,and the competition(which i didn't win)has obviously ended