If you look at the number of deaths by gun and medical malpractice, you're about 1000% more likely to be killed by a doctor's accident than you are by a gun. 30,000ish people die from guns each year while around 500,000 die from medical malpractice. So yes, in a way, doctors are more dangerous than guns.
According to the U.S. Department of Health Services, there were about 780,000 licensed physicians in the United States in 2008, so that figure seems good.
Finding statistics that everybody agrees with regarding deaths caused by doctors is a little more difficult. Dr. J. Mercola released a book in 2000 titled "Doctors are the Third Leading Cause of Death in the US, Causing 250,000 Deaths Every Year." His breakdown focuses on errors in hospitals; 12,000 deaths by unnecessary surgery, 7,000 due to medication errors in hospitals, 20,000 due to 'other' errors in hospitals, 80,000 from infections contracted in hospitals, and 106,000 from the negative effects of medications and poor prescription practices.
Doctors hold a position of respect and trust by people who go to them. Because of that they are in the position to cause us harm without our knowing about it until it is too late. It would not be the kind of intentional harm someone can do with a gun, but they can be deadly through negligence, carelessness, or a simple mistake due to tiredness. It is dangerous to have two doctors who are treating two separate conditions because often they do not know what the other one is prescribing, and will give medications that cause side effects that the other doctor may try to treat. Blindly following a doctor's orders can cause a lot of misery and may be deadly.
This may very well be one of the most ridiculous questions that I ever have had to answer. To suggest that a doctor is more dangerous than a gun is absolutely stupid. A doctor's purpose is to help and comfort people, while a gun's sole purpose is to kill others, and that is not right.
No way doctors are more dangerous than guns. While guns are used to protect people, they do so at the expense of others. Doctors, however, always intend to save lives. While the practice of medicine is an ongoing process and not always perfect, their intentions are nothing but good. Guns may intend to save others, but at what cost?
Medicine and technology have drastically extended the lifespan of people, and that includes those who research and practice it. To compare a medical professional to a firearm is simply ludicrous. First off, the gun is just a weapon. It won't hurt anybody unless somebody manipulates it. The question should be rephrased, "Are doctors more dangerous than murderers who happen to use guns."