As a former GP, currently 3rd year EM resident, perhaps I can shed some light.
First of all, the distinction between GP and FP is crucial, particularly to the folks out there who do a 3 year FP residency:
GP: General practitioner. 1 year of training gets you a license in California, and you become a GP, should you wish to hang a shingle or go to work in primary care. Unless you are VERY rural, it is unlikely you will have any admitting privileges, be able to get any type of OB malpractice, or work in any ED.
FP: Family practitioner. 3 years of training. Often less able to get OB or hospitalist type of privileges in urban IM and OB-heavy hospitals. However, 3 years of broad training certainly provides a solid base to perform safe routine OB and hospitalist work in areas that are less specialist dense.
That being said, a boarded EM physician could likely work in a VERY rural place in a full FP capacity if the place was REALLY desperate (i.e. prison or Indian reservation.)
Would likely be able to practice safely in a very basic primary care setting in a GP capacity, as it would overlap a great deal with much EM.
The basics in lipid management, hemoglobin A1C, microalbuminuria, annual pap/breast/colonoscopy, testicular exams, depression and anxiety, hypothyroidism, are things that could be learned fairly quickly, particularly if the physician was
dedicating his/her time to primary care exclusively. Like emergency medicine, much of primary care is obtaining timely referrals, and following a course of action with the assistance of a specialist.
Hope this helps, and I hope I was able to avoid stepping on the toes of any FP's.