Valenciennes are yet to pick up a win this season but struck back on the stroke of half-time through Remi Gomis before a Nene penalty secured Antoine Kombouare's side their first three points of the season.
Gameiro had a good chance to give the hosts the lead in the tenth minute after good link-up play with Jeremy Menez and Mathieu Bodmer but he fired his effort over the bar. Menez, who looked lively during the opening stages, then beat the Valenciennes defence but could not test Nicholas Penneteau in the visitors' goal.
Gaetan Bong had Valenciennes' first clear-cut effort after 33 minutes when he was played in by Gregory Pujol but PSG goalkeeper Salvatore Sirigu was equal to the Cameroon defender's effort. PSG were ahead five minutes later when Menez picked out Gameiro in the box and the former Lorient striker slotted home to open the scoring.
Valenciennes equalised right on half-time when Renaud Cohade's pinpoint cross was headed beyond Sirigu by Gomis. Pastore was introduced from the bench after 57 minutes and the home side were back in front seven minutes later when David Ducourtioux was penalised for handball and Nene converted the resulting spot-kick.