Karl Toko Ekambi s eighth goal of the season was not enough for Lyon at Nice. AFP
Guessand, who only turned 20 in July, was shown a yellow card for celebrating the crucial goal which also sent his side second in the table, eight points behind Paris Saint-Germain.
The visitors led 2-0 with 20 minutes to play after midfielder Houssem Aouar added to Cameroon striker Karl Toko Ekambi's first half effort.
The home side's comeback was sparked with nine minutes left as Algeria left-back Youcef Atal pounced to open his account for the campaign.
Guessand was brought to the fray by coach Christophe Galtier to replace Denmark's Kasper Dolberg two minutes later, for just the youngster's seventh Ligue 1 appearance.
With five minutes to go Lyon's Zimbabwe left-back Tino Kadewere was shown a red card for an aggressive tackle on Melvin Bard.
Nice, who are owned by British billionaire Jim Racliffe, equalised from the spot with a minute of normal time remaining as Atal's international team-mate Andy Delort converted from the spot.
The three points were secured after 93 minutes as Guessand nutmegged Lyon goalkeeper Anthony Lopes to send the Allianz Riviera crowd into delirium.
Later, leaders PSG head to Marseille in French football's most heated rivalry but Parisian fans have been blocked from travelling south to due recent crowd trouble in the league.
It is set to be Lionel Messi's first taste of the derby after the Argentina forward scored twice in the midweek Champions League win over RB Leipzig.
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