Comfortable on either side of the front-row, McCallum makes the switch to Scotstoun following five seasons at Edinburgh.
He made his professional debut in October 2016 in a home victory over Benetton at BT Murrayfield, before going on to make 63 appearances for the capital side.
McCallum's ability at both the set-piece and around the field have also seen him catch the eye of the national selectors, with the former Dunfermline RFC man having earned Scotland honours at both U20 and senior level.
The prop was part of a talented Scotland U20 squad that competed at the 2016 World Rugby U20 Championship in Manchester, with new team-mates Zander Fagerson, Scott Cummings and Ally Miller also part of the 28-man playing group.
He then went on to make his Scotland debut off the bench against Wales in the opening match of the 2018 Guinness Six Nations, before earning two further caps in Test matches against Canada and the USA in the summer of the same year.
A product of the FOSROC Scottish Rugby Academy, McCallum also holds a UKCC Level Two Coaching certificate for rugby.