Looking at the picture there's a door and obviously the cut out is to accommodate that. Looking at how I would approach it, I reckon it would take me three or four attempts resulting in a shedload of scrap plasterboard. But, that timber work is a very complicated solution to a very simple problem.
Use a multi too to shave off the bottoms of where the joists meet the beams, pieces of 2 x 1 around the opening above the height of the door and put a piece of plasterboard straight across, do away with all the shaping.