When I cut out my plasterboard piece, I missed that I needed to cut out a section at the bottom near the closet (the gap between the blue wall and the plasterboard). Now I have a gap around 6-8 mm which is about 13 mm deep. Is it OK to plaster, or will it not turn out well, and I should instead cut out a small piece of plasterboard and glue it there before plastering the smaller gaps?
I had similar issues after replacing a window. So I went down to the local paint store and bought some filler that was plastic-based. It could be applied thickly and it didn't sink. I also believe Flügger has something similar in their range, but in a syringe.