Substitute Gameiro tapped in from a Zlatan Ibrahimovic cross 10 minutes from time after David Beckham started a counter-attack nine minutes after coming on as a substitute.
The hosts were denied two possible penalties after Benjamin Stambouli brought down Jeremy Menez and Gregory van der Wiel in quick succession.
Ibrahimovic came close with a 20-metre snapshot that shaved Geoffrey Jourdren's left post.
Jourdren dived to his right to tip away Thiago Silva's header as PSG increased the pressure in a lively first half.
PSG, however, struggled to break the deadlock, prompting Carlo Ancelotti to substitute midfielder Clement Chantome with striker Kevin Gameiro while Beckham came in for Marco Verratti.
Beckham had an almost immediate impact as the former England captain's pass was collected by Menez, who released Ibrahimovic on the left for the Sweden striker to gift Gameiro with the perfect pass.
Four days before Barca try and impose their possession game at the Parc des Princes in a much-anticipated Champions League quarter-final first leg, PSG showed they were quick on the break.
They could be even more dangerous on Tuesday when Brazil forward Lucas and Argentine striker Ezequiel Lavezzi are expected to be back in action after missing Friday's game through injury.
French champions Montpellier barely threatened the PSG defence, which once again was perfectly marshalled by the impressive Thiago Silva.
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