In 1555, he was appointed as the coadjutor, i.e. successor, of Archbishop John Hamilton of St Andrews.
Archbishop John Hamilton died on 7 April 1571; Gavin's name is mentioned in the record of a parliament held at Edinburgh on 13 June, as "Gawan Hamilton, archbishop of St. Andrews".
Although there is no record of his formal appointment, the parliament notice appears to tell us that Gavin indeed ruled as bishop for a few months.
Gavin died in a skirmish a few days after this parliament.
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