A General Practitioner or GP doctor is a doctor that works with both children and adults while an internist is basically an adult pediatrician.
The internist only deals with and treats adults only while the GP or General Practitioner is basically an adult doctor and a pediatrician all in one.
So that is the main difference between a GP and an Internist doctor.
Both GP and Internist doctors are primary care physicians but like I mentioned internal medicine doctors work with adults only while the GP doctor cares for both adults and children.