I'm not an expert with this. But how old are the chickens? Have you chance anything: there food, wood chips tried a diffrent treat? Just some type of change in there diets. How's the weather where you live? Is it too hot?
there are a few different ages from 2 monthes on up to 1 year, some of mine has this problem, and a few of my cousin's has this problem. nothing has changed, the weather has been around the 70s and 80s sometimes 90s.
i had somthing similar go through my whole flock one at a time this winter. their eyes would be kind of guled shut and bubling, they couldn't see at all and would stop eating and drinking as a result and as you described they would gasp for air. it took a few days to for each bird to get over it. I lost 4 out of 30 to it. but most did get over it on their own. good luck with yours.
It's probably coryza or sinusitis. I use tylan and inject it in the meaty part of the breast. It normally takes 3 to 5 days to dry it up. Dosage depends on body weight. Like a silkie i would give 1 1/2 to 2 cc's every other day until symptoms are gone. Basicly this is a type of cold and a chicken can't blow it's nose. Any other questions pm me. I am glad to help.