Fenbendazole is the recommended treatment for gapeworms, but not sure on the dosage. You probably need to treat her right away it sounds like, they can totally block the trachea/air passage of your hen, so she is probably NOT getting enough oxygen.
This is what I did, but I cant recommend it, k? Your taking your chances, since its not bona fide for hens.
I actually was desperate with mine and used Panacur, which is a doggie dewormer made of fenbendazole, and just used very little in their water (maybe an 1/8 package, it was for a 20 lb dog, so I tried to go by about a 3 lb chicken?), and it worked, but I dont wanna recommend anything that would hurt your chicken. I was desperate as they carry it at our 24 hour
Walmart in the doggy form, I didnt have time to order it, my feedstore didnt carry it and I thought I was gonna lose my baby, so I did what I thought was necessary.