In November 1721 he traveled to Aberdeen and acted as Bishop Archibald Campbell's vicar-depute. Gadderar supported the practice of primitive 'usages' in the diocese, which brought him into a dispute with the College of Bishops at Edinburgh. After the resignation of Bishop Archibald Campbell in 1725, he was made Bishop of Aberdeen, remaining there until his death.
In November 1721 he traveled to Aberdeen and acted as Bishop Archibald Campbell's vicar-depute. Gadderar supported the practice of traditional 'usages' in the diocese, which led to a disagreement with the College of Bishops at Edinburgh. After the resignation of Bishop Archibald Campbell in 1725, he was appointed Bishop of Aberdeen, remaining there until his death.