I just saw your comment about the sneezing. It'll be a whole series of sneezes - like 5 to 10 or so - and there will be periods of the classic gape, meaning they extend their necks out with their beaks wide open and make a caw-gasp-cough sound. It sounds horrible, like they're really struggling to breathe. Also note that when they sneeze there's no discharge.
The fact that now all your sexlinks are showing signs makes me think that either it's not gapeworm or they all got infected at once... We have about 40-50 chickens at any given time and I've never had more than one case of gapeworm at a time. It's a really easy cure if it is, though.
Let us know how it goes! Good luck!