Depends imo... gill curl "man-made" is usually a physical symptom of poor husbandy ect... so will gill curl alone kill a fish? no but it usually means the fish has been in poor health ect for a very long time... but as already pointed out it can happen in perfectly otherwise healthy fish ( I don't 100% buy it but I've seen gill curl in newly shipped in cats that before the bag was opened they where fine. so that sudden ammonia spike was enough to "curl" them but realistically it likely hasn't done the damage a cat that has been kept in prolonged poor conditions whos curl may look far less "bad" actually has.) And TBTB also made a excellent point these fish tend to suffer from infections ect of the gill tissue more often, but I also think it has to do with an over-all lack of proper husbandry. ( I kinda view it like we do our livers, I know some people that can drink a lot and are perfectly healthy and some people who can't or don't drink much because of genetics or they just don't have the liver function. hey I'm from Wisconsin!) No 2 fish "stunt" the same way... but TSN's aren't small cats... its imo easier to stunt them then an RTC.
Good care/Husbandry will often fix it I def agree.. not always but often enough its worth a shot.
gill curl is also seen in other large predatory species.. ime the common denominator is poor water quality/or inadequate space (stunting) I've seen it in cichlid, catfish, gar, a GG, arrowana...