Fill a gap wider than about 1 inch, either on a flat or corner seam, with a strip of drywall. Cut the strip, if possible, from the paper-covered edge of the sheet with a .
Jimmy's corner bead solution is probably easier and I upvoted it. . I cut a narrow strip of drywall to fill the gap as much as possible, then mud.
Create a seamless corner by finishing your drywall with corner beads. Here are If you've ever dented metal corner bead with a misplaced hammer blow during.