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, Gadderar traveled to Aberdeen and served as the vicar-depute for Bishop Archibald Campbell. He supported traditional practices within the diocese, leading to a conflict with the College of Bishops in Edinburgh. Following the resignation of Bishop Campbell in 1725, Gadderar became Bishop of Aberdeen, holding that position until his death.