Repair long narrow gap in plaster
Since it's too small to fit a piece of drywall, I would fill the gap with setting-type joint compound embedded with fiberglass mesh.  It may take two or three coats to fill the gap.  After it's filled, finish the two sides like you would if it were filled with a piece of drywall:  Tape the patch with paper and feather out to the plaster.  Then put on a finish coat of compound.  Many folks prefer to use the pre-mixed stuff to finish, but you can use the setting type you used to fill the gap.