Can general practitioners help with gum recession?

Yes but limited. Receeding gum lines is usually due to poor gum health. This occurs from plaque and tartar building up on the tooth at the gum line causing an inflammatory response (gingivitis). Primary care physicians generally leave this to their dental colleagues, but can recommend good oral health, including daily flossing. Some may also prescribe an oral rinse (periodex) to help. See your dentist regularly.
Yes. By reminding patients of the importance of seeing their dentist at least 2x/year as well as regular brushing and flossing.
See Periodontist. Gum recession indicates serious damage to not only the gums but also the underlying bone support. GP physicians have little training or understanding in diagnosing or treating gum disease. GP dentists have infinitely more training and understanding, and also know when to refer patient to a gum/bone specialist, a Periodontist, who is best equipped to treat gum disease.