James
There is no requirement for a claimant to have a GP but it may limit their ability to provide up to date medical evidence, it will also depend on their conditions, so childhood conditions that persist into adulthood may have supporting evidence from some time ago but may not indicate how an adult might be limited, recent conditions may have up to date evidence but might not relate well to the PIP Descriptors.
Nothing on this board constitutes legal advice - always consult a professional about specific problems