Don' t think the club will want to let him go, not without a buy back clause. He's still young, and he is a promising keeper. My concern is that keepers generally need to play to improve, we saw the same thing with Fraser, sitting on the bench (or not even on the bench) helps no-one.
Depending on what happens with the other keeps in the squad, I'd be looking to let Gunn go out on loan, possibly down to the championship for a season, and get some games under his belt.