Solidifying a spot in the top four, it was their sixth straight win and in the process halted the Lions’ winning streak at nine.
Although Brisbane was wayward for much of the day and could have built a greater lead, the Giants were incredible in the final quarter.
Former No.1 pick Aaron Cadman kicked three goals for the term and youngster Darcy Jones added two as the visitors ran in waves to overpower the home team.
Tom Green (28 disposals), Finn Callaghan (24 and a goal), James Peatling (20 and three assists) and Brent Daniels (16, a goal and two assists) were all influential.
Coleman Medal leader Jesse Hogan added three goals to keep his team in the contest.
Despite the inaccurate kicking, which included hitting the post three times, the Lions built a 30-point lead at the first change.
However, a new quarter brought an entirely different complexion over the contest as the Giants ramped up their intensity to start the second and banged home three quick goals to wipe away most of the deficit.
Lachlan Keeffe lifted in the ruck while Green and Callaghan got busy through the midfield, all playing their part in the dazzling comeback win.
The Giants next face the Freemantle Dockers on Saturday from 1.45pm at Engie Stadium.
Giants wow in comeback
TRAILING by 30 points at quarter-time and still by 15 early in the last term, the GWS Giants kicked the game’s final six goals to defeat the Lions 13.4 (82) to 8.16 (64) at the Gabba in Brisbane on Saturday.