Gurgling sounds like she is sick.
Can you have her tested?
Mycoplasma Gallisepticum (MG) and Infectious Bronchitis (IB) are somewhat common as are several other respiratory illnesses. Depending on what it is, it can be contagious and affect you existing flock.
You did the right thing by keeping her separate. Practicing biosecurity between your flock and the quarantined bird will hopefully decrease spreading any contagions. Some illnesses/symptoms can be treated successfully with antibiotics, but the infected bird will be a carrier for life.
After careful consideration and research hopefully you will be able to make a decision of whether or not it is worth the risk of introducing them to your flock.
http://edis.ifas.ufl.edu/ps044
http://www.thepoultrysite.com/disea...tion-mg-chronic-respiratory-disease-chickens/
http://www.thepoultrysite.com/diseaseinfo/78/infectious-bronchitis-ib/