Traditionally, the term "GP" or "general practitioner" is an old phrase that often referred to those who would do a general internship (one year), then practice medicine. These physicians would get licensed, would not have any board certifications, and would be generalists with no specific training in any ABSM specialty. Those days are over.
Many laypeople may refer to internists and family physicians as "GPs" - but that is technically an incorrect use of the term. Today's family physicians go through a 3-year residency in family medicine and have the opportunity to get board-certified in family medicine, which is an ABSM specialty.
The same is true for internists (internal medicine, 3-year residency)