Finally got my GH3 and found a quick fix for the loose card slot door. It's VERY easy.
Circled is the actual contact point of the locking mechanism. These indentations on the metal arm slide over the door hinge when you close the door and hold it shut.
Simply remove the metal piece with a screwdriver and give these little arms a slight bend towards where the hinge will be when re-attached. Just a VERY SLIGHT bend at the circled area is all you need - this photo was taken AFTER I bent mine.
When re-attaching the piece, take care not to strip out the screw hole.