If half of his tail is missing you either have something picking at him, a water quality issue, or he has some type of disease. Combinedwith the laying on the side and gasping, I'm leaning toward a water quality issue. I advise that you invest in a set of liquid test kits covering ammonia, nitrite, nitrate, and pH. You can find the API Saltwater Master Test Kit online for about $20 or at a LFS
for around $32. The test strips while appearing convenient are often grossly inaccurate.
Royal grammas are normally pretty hardy fish. They don't stray very far from their homes, but not responding to food for 4 days is out of character for them. I'd do a water change in any case.
Any salt-like spots on the fin or body?