GWS coach Adam Kingsley hailed Lachie Keeffe's match-winning goal as just reward for the club favourite after his set shot with one minute left clinched a thrilling three-point victory over Melbourne.
Giants captain Toby Greene produced a huge final term as the visitors came from three kicks down in the second half to emerge 11.11 (77) to 11.8 (74) winners at a rainy MCG on Sunday.
Skipper Greene was inspirational with 23 disposals, five clearances and three goals and midfield star Tom Green returned from injury with a bang, tallying 35 disposals and nine clearances.
Finn Callaghan (31 touches), Lachie Whitfield (34) and Stephen Coniglio (three goals) were also influential, but it was Keeffe who was the hero in the final minute.
After being beaten by Melbourne ruck star Max Gawn, Keeffe rolled the dice late in the match and pushed forward to mark 40 metres from goal.
His set shot sailed through to put the Giants in front for the first time since early in the third quarter.
Moments earlier, Melbourne debutant Aidan Johnson outmarked Harry Himmelberg and could have sealed victory for his side, but pushed his set shot wide.
The Giants capitalised with an end-to-end play that resulted in Keeffe's unlikely match-winner.
"Lachie fought all day and got rewarded with a moment that he took," Kingsley said post-match.
"It was a beautiful kick, great finish.
"It's really important for him. He's one of our favourites."
It was a second win from as many games for GWS, who destroyed Collingwood in their season opener and are expected to be in flag contention this season.
Melbourne's season-opening heartbreak came despite a strong return from Christian Petracca, who had 27 disposals and two goals in his first game since suffering serious internal injuries midway through last year.
Clayton Oliver (25 disposals) and Jack Viney (27) were also strong performers in a reunited midfield with Gawn, who had 46 hit-outs and six clearances.
But the Giants got on top at stoppages in the decisive final term, with Greene and Green spearheading a 16-5 advantage in clearances.
"We found ourselves in a pretty tough game, so to fight our way through that was really important," Kingsley said.
"We think Melbourne's going to be a really good team so to compete with those guys, particularly at stoppage in the last quarter, after the third quarter was a little bit worrying, I was really proud of our effort."
Melbourne fielded six new faces, including five AFL debutants and inexperienced recruit Harry Sharp, and coach Simon Goodwin said there was "a hell of a lot to like" in his side's first-up performance.
"You always win in some manner and we won in a lot of manners today, and we'll certainly live in that space," Goodwin said.
"There's still lots to learn in the last 25 minutes of the game but we did a lot right as a footy club.
"We walk away without the result but we walk away emotionally as a club feeling like we're on the right path."
GWS forward Brent Daniels was substituted out with a bruised hip but Kingsley expects him to be fit to play after the round-two bye.