The framing on my house is warped in such a way that I have several places where I have large gaps between the window/door architraves and the frame and the plasterboard just flaps around. I also get the feeling that whoever hung the plasterboard thought that screws were really expensive.
This one window already has a some gap filler in it, but the gap has widened. Two options I can think of are to insert wedges to push the plaster back then use more gap filler, or pull the plasterboard forward and glue it to the back of the window frame.
Or is there a better idea I can't think of?