Chances are, she has a respiratory disease. Other things in the envirorment, such as ammonia and dust, may also cause sneezing and gurgling, but if she's had it for a while, I would lean to a respiratory disease.
The unfortunate thing about most respiratory diseases is that they never really go away. Viral respiratory diseases can't be treated by antibiotics (this may explain why antibiotics didn't work for you). Even respiratory diseases caused by bacteria don't truly go away and can't really be treated. Respiratory diseases almost always make birds carriers, for life, and these birds may show symptoms in times of stress. They can also pass it on to other birds in the flock.
Your bird may eventually seem to get over this disease. But, it won't be gone and she may show it again at another point in her life. Your other birds have unfortunately probably been already exposed. They may or may not show symptoms, depending on their stress level and resistance.
Having a respiratory disease in the flock isn't a death sentence. Your birds can still live mostly normal lives, and they may get over it and never show symptoms again, even though they still carry it. You'll just have to be careful about bringing new birds in to the flock, as they will be exposed as well. You should maintain a closed flock, as in not selling these sick birds to anybody else, as that could introduce it into other flocks.
This could all be speculation; your bird may not actually have a respiratory disease. I just think that it is the most likely.
Here are some links on respiratory diseases in chickens:
http://www.dummies.com/how-to/content/respiratory-illness-in-chickens.html
http://www.thepoultrysite.com/disea...ction-mg-chronic-respiratory-disease-chickens
http://www.nda.agric.za/docs/Infopaks/default.html
http://edis.ifas.ufl.edu/pdffiles/ps/ps04400.PDF