It was the first round of the Highlanders First XV competition, played in wet conditions at King's.
Gray kicked accurately to the corners and kept McGlashan pinned in its own 22m for long periods in the first spell.
King's controlled possession and territory and led 3-0 at the break when first five-eighth Gifford Henry kicked a penalty goal.
But it was a different story in the second half when McGlashan attacked through its forwards, who were held up over the line twice.
Loosehead prop Hayden Fleury crashed over for an early try in the second spell and Henry's conversion gave King's a 10-0 lead.
It needed desperate defence for the rest of the game to keep a rampaging McGlashan side out. Accurate tackling by Henry and openside flanker Ben Johnson helped keep McGlashan scoreless.
The key player for McGlashan was halfback Tom Dickison, who had an outstanding game.
The forwards played well as unit, with hooker Sam Pringle throwing accurately to the lineout and lock Lachie Angland providing the ball.
Hooker Ron Carter was in dynamic form and scored four tries in wet conditions when Southland Boys' High School scored seven tries to beat Kavanagh College 41-14 at Kettle Park.
Most of the play was kept in the forwards and loosehead prop McRae Sanderson did his job with his driving play.
The best Southland Boys' back was centre Kenau Kuhurkara, who ran hard and made two breaks that led to tries.
In a high-scoring game in Alexandra, Waitaki Boys' High School scored five tries to beat Dunstan High School 34-28. Dunstan scored four tries and gained two bonus points.
Newcomer Menzies College made a successful start in its first appearance in the competition with a 12-8 win over South Otago High School.