Sydney kicked eight goals to three in the second half, with thunder and lightning around the inner-western Sydney venue in the last few minutes and the players splashing through puddles and surface water.
There were a couple of injury concerns early in the game, which Sydney won 14.13 (97) to 8.11 (59) on Thursday.
Swans star Errol Gulden wasn't sighted after quarter time after tweaking an ankle.
Gulden, who missed a large chunk of last season with an ankle issue, kicked the game's first goal from a 50-metre set shot before being withdrawn for precautionary reasons.
Swans officials were adamant he will be available for next Thursday's season opener against Carlton.
GWS spearhead Jesse Hogan spent some time off the ground in the first half with what appeared to be an ankle issue, but returned to the ground and played out the game.
Grundy showed plenty of athleticism at the centre bounces, with the amended rule placing a premium on jumping rather than physical contact between ruckmen.
He amassed 15 touches, four clearances and 33 hitouts and showed plenty of run and effort.
In tough conditions for key forwards, Sydney's superstar recruit Charlie Curnow had a joint game-low six touches.
He booted an impressive goal on the run from close to 60 metres early in the game before the rain became a factor and added a second early in the second quarter.
The Giants' big recruit Clayton Oliver gathered 19 touches and a well-taken goal.
Sydney won three of the four quarters, with GWS kicking half of their goals in the second term.
Five straight goals either side of quarter time gave Sydney a 38-point lead.
GWS rallied with the last four of the half, two goals apiece to Hogan and Jake Stringer, to get within 12 points.
Sydney booted the first three of the second half to regain control and never relinquished it.
Grundy apart, Nick Blakey, Angus Sheldrick, James Rowbottom and Isaac Heeney were other strong contributors for Sydney.
GWS captain Toby Greene was the standout player for his team, tallying 26 touches, eight tackles and six clearances.
Stringer kicked a game-high three goals.
The Giants were without several injured players, but star midfielder Finn Callaghan told Fox Footy and Kayo he expected to play in their opening game against Hawthorn, despite missing their two practice games.
Sydney recruit Malcolm Rosas Jr was reported for abusive language late in the game, while the MRO may scrutinise high contact from GWS ruck Nick Madden on Swans speedster Chad Warner.
