The steps where Gil sits waiting for a vintage Peugeot to take him back to the 1920s are on the left side of Saint-Étienne-du-Mont, just behind the Panthéon.
Many people look for them in Montmartre; they are on the Left Bank, in the 5th arrondissement. The production notes name rue de la Montagne Sainte-Geneviève as the street where Gil goes at midnight.
The Paris tourist office’s Ciné-Balade walks the Montagne Sainte-Geneviève, the Panthéon and Saint-Étienne-du-Mont on its way from Odéon.
The steps are on the left side as you face the church, just behind the Panthéon. It is a working church: sit, but keep the way clear.
Where this comes from

Paris je t’aime, the Paris tourist officeMidnight in Paris: a cinematic stroll (Ciné-Balade)
The Left Bank · From €19 · choose a date when booking
A 2 hour 15 minute walk from Odéon to the Panthéon and on to Saint-Michel: Shakespeare and Company, Polidor, the Montagne Sainte-Geneviève and Saint-Étienne-du-Mont, with clips played on a tablet. In French and English; meet at the Danton statue.
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