Hawthorn selected Dewar with pick 55 in the 2010 AFL Draft. He was drafted as a tall defender from Claremont, and represented Western Australia at the 2010 AFL Under 18 Championships. A foot injury finished his 2011 season whilst developing at Hawthorn's affiliate club Box Hill.[1]
He spent 2013 developing in the VFL playing for Hawthorn's affiliate side Box Hill and was a member of their premiership winning side. Dewar made his AFL debut against Essendon in Round 2, 2014 as a late replacement for Sam Mitchell.[2]
Dewar was used as a defender for the Hawks, he had a booming right foot kick and had plenty of dash off the half back line. Finding it difficult to break into a premiership winning side, Gus managed to play half a dozen games a season. In 2016 he started the year in the side only to drop out and ended the year with the Box Hill Hawks. At the conclusion of the 2016 season, he was delisted by Hawthorn.[3]
In 2022, as a COVID-19 top-up player on the West Coast Eagles list, he played one game against the North Melbourne Kangaroos.
Dewar now plays for Subiaco in the WAFL where he has won three premierships.