TO clarify, the GP does not have to mention the name of the school: after all, GP's are not school inspectors and cannot reasonably be expected to know what local schools provide what.
What the GP does have to do is to specify that because of X condition having Y effect this child needs Z type of school. It is then your job to prove that the only school that offers Z is the one you are appealing for.
In our case, dd had EDS syndrome; our argument was that she would need: a school with wheelchair access for her bad days, a small school so she could walk on her good days, a local school so she would not have to tire herself out by travelling, and a school attended by at least some people she knew to manage the anxiety surrounding having to explain her condition.
I gave a note to the GP beforehand explaining that these were our arguments and asking him to specify, in as far as he felt he could do so of his own knowledge, what her needs were re wheelchair access, need to be encouraged to walk on good days, anxiety concerning her condition (here we also submitted evidence from a therapist) and difficulties with travelling.
We then turned up at the hearing with plans of the local schools which showed quite clearly that only one school met the requirements specified in the GP's letter. We won the appeal.
The GP has said since that he is always happy to supply evidence of what he has observed, but that it is difficult for him to know exactly what any one situation requires so he is grateful if we explain it to him.