In my experience, halfmoons aren't prone to cloudy eye or cataracts although they do tend to have eyes that bug out more than other types (making some people worry that their betta has popeye). If Gabe has cloudy instead of cataracts, the clean water and AQ salt should help but if the cloudiness doesn't go away, I'd say there's a good possibility it's cataracts or another age-related problem.
Has he been more willing to eat at all? He might need to stay in the 3gal for a while, even after he gets better, just because his tail is so big. At his age, it must not be easy carting that tail up to the surface to breathe. He's a gorgeous guy. What a find. :)