Gallinari overcame an 0-for-10 performance from the field in the first half to nail the decisive shot with 35 seconds remaining in the second extra frame, finishing with 21 points.
Kenneth Faried supplied 18 points and 17 rebounds as the Nuggets used an 11-0 run down the stretch to close out the triumph.
The Warriors fell despite 23 points from Klay Thompson and 22 off the bench from Carl Landry.
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'
Beyonce flaunts bikini body, Blue Ivy in vacation pics