Steve Nash had five points on 2-of-9 shooting and added eight assists in just over 26 minutes for the Suns, who earlier Thursday waived guard-forward Mickael Pietrus.
After opening the game 6 of 24 from the floor in the first quarter, the Suns recovered and led 60-56 on Markieff Morris' 3-pointer with 7:40 left in the third quarter.
But Gallinari and the Nuggets took over from there. Gallinari converted a three-point play, Chris Andersen sank a pair of free throws and Brewer hit a 14-foot turnaround jumper with 5:04 to go in the third for a 70-60 lead.
Phoenix rallied to within six at 77-71 on a three-point play by Brown with 13.5 seconds left in the period, but Gallinari hit a fade away jump shot at the third-quarter buzzer and the Nuggets coasted the rest of the way.