I can believe that a lot of GPs have not heard of it. A couple of years ago I trained in CBT. There were some counsellors on the course who had being doing it for 20 or 30 years. Not one of them had heard of Paruresis.The problem is that because the condition is driven by embarrassment people rarely speak to counsellors or even their GP.
It did however once appear on embarrassing bodies. The doctor (I forget his name) recommended a course of CBT.
Although many counsellors may not have heard of it they will soon recognise it as an anxiety disorder and know how to treat it (especially if they are CBT trained)