The problem of GERD
can be the cause of this chronic sore throat
Since after investigation nothing is coming up means that your problem of GERD and gastritis is causing this problem.
In cases of GERD and gastritis, the acidic gastric eflux reaches the throat can this cause a sort of chemical injury
to the throat and as the GERD and gastritis is chronic condition this continue for long time and the chances of chronic sore throat is there.
So I would suggest you to consult your doctor for this proble of GERD and gastritis and once this is cured the problem of sore throat will be contolled.
Thanks and take care.