On the vertical, you've got support from the stud, so you could rip a piece to fit in there, screw it down then tape and fill. If you used an edge where the paper wrapped around, it would be easier to make it look decent. But, you could also pick up a U-channel end cap for drywall and fit it over that piece's edge.
As to the horizontal strip at the top, you could probably do the same thing, but you might want to cut back some more drywall so that the new filler piece could be larger...that narrow strip wouldn't leave you much room to anchor it without splitting it, or keeping it from rocking.
Because the expansion/contraction of the fiberglass walls and the drywall won't be the same, you may want to keep the drywall slightly away from the fiberglass and use some while silicone caulk there to flex - the drywall mud would crack. Note, a foam filler rod stuffed into that joint prior to caulking makes things last longer and use less caulk.
