To treat gapeworms give 0.23 ml (1/4 ml) per pound of weight for 5 straight days. Why do you think it is gapeworm? Where are you located? It is very rare. Respiratory diseases are much more common and can cause gasping. Do you have a video or pictures? Videos must be uploaded to YouTube or Vimeo.
If it is gape worm, here are the directions I am currently following for my chicken flock using the exact same product you have purchased. I had one hen die from gapeworm. Very typical presentation of symptoms so there was no doubt. Unfortunately I have seen this in the past so I am familiar with the infection. Some of my other chickens have presented with dirty bottoms since then (runny poo stuck in their feathers around vent area), but thankfully no one has been gasping or gaping. http://www.poultrydvm.com/condition/gapeworms