I felt like my GP was really only good for getting the ball rolling: I asked her to refer me to a psychologist for testing and she did. To be honest, I think you could skip the GP step and just meet with any clinical psychologist to discuss with that person your concerns about ADHD. Their office should be able to tell you if they can do ADHD testing before you go in. Some psychiatrists may test as well. The difficulty, of course, is that psychologists are often out of pocket or are paid at a reduced rate by insurance.
After the testing is done many GPs are willing to do med management, though it's a pain, since the Federal government requires prescriptions to be for no more than 30 days and you can't refill it more than three days ahead of time. Personally I have my neurologist manage it, but I have epilepsy and it makes sense to me to go to her for all of my brain medications.