Put another layer of drywall on each wall to take up the gap

The picture looks good. Test them by driving a nail into one of them. If the mail goes in as you would expect, they’ll be fine. If it feels soft or spongy, they will need replaced. I used some tar paper on mine to cover the top and folded it over about 50% of the sides to keep water off the joists. The joist tape works fine but it’s very expensive if you have a large deck and only protects the top.