I would probably take the panel down and try again.
Even if you used fiberglass mesh tape when you tape the joints, it'll be hard to finish the surface, and you're probably going to experience premature cracking, etc.
You could try filling in the gap with one of several materials:
- foam backing rod (usually used for caulking large gaps)
- expanding foam insulation
- thin cut wood/plywood
But I'm not sure how the mud would dry over material like that. I'd imagine that it'll still be more prone to cracks than a tight gap. If you are going to try to fill the gap, a harder material would probably work better than a softer material like foam, simply because it would provide more support against bumps and scrapes.