Gameiro, who had only played 71 minutes in the league this season, scored a first-half double as PSG confirmed they had reached cruising speed after a stuttering start.
"We were not that good in the second half, we started to get a little bit tired, it's normal," goalkeeper Salvatore Sirigu told Canal Plus. "The good thing today is the result. There is still room for improvement but we keep cool."
"There are moments like that in a career," he told reporters. "Whether I start a game on the field or on the bench, I try to seize opportunities."
Javier Pastore's delicately chipped pass into the path of Gameiro allowed the latter to make it 1-0 with a low shot after 11 minutes. Gameiro doubled the lead 12 minutes from the interval when he latched on to a Maxwell cross from the left.
Gameiro's double made up for Zlatan Ibrahimovic's failure to score for the first time in seven PSG appearances.
PSG, who have scored 10 goals from their last three games, face a couple of important challenges next week as they travel to Porto in the Champions League before the "classic" with Marseille.
Montpellier's second victory of the season put them on eight points, 10 adrift of Marseille.
Lorient stayed fourth on 13 points after Gilles Sunu's late goal salvaged a 1-1 draw at Evian Thonon Gaillard.
