Jallet chested down a Jeremy Menez cross and fired a volley into the net one minute into stoppage time to put PSG on 59 points from 28 matches.
PSG, unbeaten in all competitions since coach Carlo Ancelotti took over from Antoine Kombouare in December, lead
Montpellier had Vitorino Hilton sent off in the 42nd minute and were holding on until Benjamin Stambouli was also shown a straight red card with Andre Luiz converting the resulting penalty shortly after the hour to hand the visitors their first defeat in 2012.
PSG were also heading to their first loss of the year when Jallet salvaged a lucky draw following a mediocre performance by the leaders.
Strugglers Caen went ahead nine minutes into the second half when Pierre-Alain Frau connected with a Livio Nabab cross and scored from close range.
Argentine midfielder Javier Pastore equalised two minutes later by putting a poor clearance into the net but the hosts took the lead again in the 71st minute with Thomas Heurtaux heading home from a corner kick.
Keeper Stephane Ruffier denied Michel Bastos and Anthony Reveillere with superb saves, then blocked a Lisandro Lopez volley, but there was nothing he could do when Gomis unleashed a 25-metre rocket with nine minutes left.
Lyon now have 46 points and trail third-placed Lille, who occupy the Champions League play-off spot, by one point before the French champions host neighbours Valenciennes on Sunday.
St Etienne are fifth, also on 46 points, but trail on goal difference.
Four days after qualifying for the Champions League last eight at the expense of Inter Milan, it all started well for Didier Deschamp's side as they took the lead thanks to a Loic Remy penalty after 11 minutes.
Midfielder Younousse Sankhare made it 1-1 midway through the first half from close range and Gael Kakuta converted a penalty 11 minutes from time after Nicolas Nkoulou was sent off for a foul on Koro Kone.