Devon Smith kicked a goal with 16 seconds remaining to ruin the veteran Bulldog's last match, while sending their own retiree Josh Hunt out a winner.
With 30 seconds to play, Jake Stringer dropped a straight-forward mark inside 50, with the GIANTS able to rebound and snatch an incredible 18.7 (115) to 15.19 (109) win.
There was no fairytale finish for Giansiracusa at Etihad Stadium, who had three shots for goal in the final five minutes but managed just a point.
Bulldogs skipper Ryan Griffen was best on ground with 38 touches and 13 clearances but the GIANTS young brigade again stood up.
Dylan Shiel and Toby Greene were industrious, while Will Hoskin-Elliott's five goals led all-comers.
GIANTS coach Leon Cameron said the manner of their sixth win made it all the sweeter.
"To win a really good close game with our group is absolutely fantastic," Cameron said.
"It's not affecting the ladder but it reinforces we're on the right track."
"We'll hit the ground running in the pre-season knowing if we bring that sort of effort we can compete against anyone."
If GWS led early by taking its opportunities, with 4.0 to 2.4 at quarter-time, it surged in the second term on their talent.
The GIANTS kicked six goals in ten minutes, with a sole Giansiracusa major the only speedbump as they surged to a 33-point lead.
They blew it all after the main change, with Stewart Crameri helping the Dogs seize control.
With the match on a knife-edge, the GIANTS built a 14-point lead which was again dismantled by Marcus Bontempelli and Crameri.
The match was tied for the final three minutes, with the Bulldogs unable to make the most of their chances.
That was the story of their day, with many more entries to their forward line but none of the GIANTS' potency.
"If there was a mistake to be made, we found it," said Bulldogs coach Brendan McCartney.
"We invented a few new ones."
McCartney gave his side no credit for mounting two comebacks.
"So we should, we were the more experienced team out there today... we should have been able to conjure something up."
WESTERN BULLDOGS 2.4 5.8 11.14 15.19 (109)
GREATER WESTERN SYDNEY 4.0 10.4 12.6 18.7 (115)
GOALS
Western Bulldogs: Stringer 4, Crameri 4, Giansiracusa 2, Bontempelli 2, Minson, Griffen, Liberatore
Greater Western Sydney: Hoskin-Elliott 5, Stewart 3, Treloar 2, Palmer 2, Boyd, Williams, Greene, McCarthy, Smith, Kelly
BEST
Western Bulldogs: Griffen, Murphy, Giansiracusa, Stringer, Liberatore, Boyd
Greater Western Sydney: Hoskin-Elliott, Shiel, Treloar, Shaw, Palmer, Stewart
INJURIES
Western Bulldogs: Nil
Greater Western Sydney: Williams (lower leg)
SUBSTITUTES
Western Bulldogs: Koby Stevens replaced Mark Austin in third quarter
Greater Western Sydney: Cam McCarthy replaced Liam Sumner in the third quarter
Reports: Nil
Umpires: Fisher, Hay, Harris
Official crowd: 14,725 at Etihad Stadium
