A former pupil at Auchinleck Academy, Gavin has represented Glasgow at under-16, under-17 and under-18 levels and was in the Scotland under-18 squad in 2011/12 and 2012/13.
He made his international debut for Scotland under-18 against Wales in 2012 and continued in the squad throughout the FIRA tournament that year, racking up a total of 27 points (5 tries and 1 con). He was again part of the Scotland under-18 squad into 2013 and on to the FIRA tournament in Grenoble, where his conversion and two penalties helped seal the opening win over Wales and ensure a top-four finish for the Scots. His under-20 international debut came against the Irish in Athlone, in the 6 Nations opener in January 2014. He scored a penalty and a conversion, at stand-off, in the defeat to Wales at Colwyn Bay. In the final game of the 2014 IRB JWC, against Argentina, he converted his own try.
His international sevens debut came in December that year when he was called up to play in the Dubai and South Africa legs of the 2014/15 HSBC Sevens World Series. His sevens try came against Wales at the 2015 London 7s against Australia.
He was a Scottish Cup winner with Ayr under-16s in June 2011 when he also picked up the award for player of the match. He won the Scottish Schools Football Cup in 2010 with East Ayrshire and was Scottish Schools under-14 badminton double champion in 2009.