If you do not live in a tropical area and your chicken is showing symptoms of gasping or constantly adjusting their crop, they likely do NOT have Gapeworm! It's more likely sour crop or a respiratory infection, both of which will not be treated with worming medication. Your first thought at seeing these symptoms should be to worry about your birds ability to breathe and if it is contagious to your flock. Respiratory illnesses or canker are the most likely culprits if multiple birds are affected.
A lot of new chicken owners see those symptoms and read online about gapeworm but it really isn't that common. If you have a chicken who passed away who showed those symptoms it's incredibly easy to confirm it at home since the Gapeworms take up residence in the trachea. I even see people just assume that's what killed their chicken without checking.
So to reiterate, unless you are in a tropical climate where your birds have access to lots of earthworms and slugs they likely do not have gapeworms, especially if it is multiple birds displaying symptoms at once. Gapeworm is also easily identified by an experienced vet as all it takes is checking the trachea.