Gill hooked fish have the odds stacked against them but like any injury it's all in how bad it really is and the species of fish coupled with the water conditions at the time. We had a bullhead last for years in a farm pond with a 22 caliber pellet lodged deep in it's skull. Fellow up the road caught it, shot it, found it alive when he got home that night and released it in a small pond to be harvested later. Turns out it lived for years. Other fish aren't nearly so hardy.