PSG entered the game four points adrift of Lille following the league leader's earlier 2-0 win at Dijon and took time to get going, even if Ander Herrera did come closest to a first-half breakthrough when striking the crossbar.
However, Gravillon was dismissed for bringing down Mbappe early in the second half at the Parc des Princes and the PSG forward brushed himself down to convert the subsequent penalty.
Kean fired in a second goal nine minutes later and PSG was under little threat from that point on as it climbed to second, ahead of Lyon, going into Monday's (AEDT) huge top-of-the-table showdown with Lille.
Yoane Wissa got in behind Danilo Pereira early on but the makeshift PSG defender blocked the shot - sustaining an injury in the process that forced him off - and Sergio Rico had a simple save to make.
Lorient put three goals past Nimes last weekend to end a four-game goalless losing run and they wasted another good chance to open the scoring in Paris, Terem Moffi racing clear but pulling his shot wide of the far post.
PSG hit the crossbar late in the first half through Herrera's curler, though it only had to wait until six minutes into the second period to open the scoring thanks to Mbappe's 11th league goal of the campaign.
Mbappe burst past Gravillon and was brought down by the visiting defender, who was issued a straight red card and further punished by his opponent drilling the penalty underneath Paul Nardi.
A big chance went begging for Mbappe almost instantly but PSG again carved its opponent open with an hour played, a fine team move culminating in Rafinha releasing Kean to convert past the onrushing Nardi.
Layvin Kurzawa joined Herrera in hitting the crossbar at the end of a counter 10 minutes from time but it mattered little as Lorient could not muster a response, Adrian Grbic coming closest to beating Rico when hitting the post late on.