Gregorian mass in Paris

You can look for concerts of this type on this website:
https://www.classictic.com/en/

Here are some wild guesses - sorry, it's very hard to tell, given the information you supplied...

https://en.wikipedia.org/wiki/Saint-Julien-le-Pauvre
https://www.eglise-saintgermaindespres.fr/
https://www.classictic.com/en/paris/eglise_saint-s%C3%A9verin/29/
https://www.collegedesbernardins.fr/

Also try this Latin Mass Directory.

This may be Saint-Gervais-Saint-Protais
13 Rue des Barres, 75004 Paris.

Mass and Vespers are broadcast from Notre Dame Cathedral via www.ktotv.com
If you look up the "grille des programmes" at ktotv you will see "Lauds from St. Gervais" listed at 7.00 AM Tuesday to Saturday. The actual website is jerusalem.cef.fr There should be videos of previous broadcasts somewhere, say if you go back in the programme list.

This is what ktotv says,
"Du mardi au samedi, KTO diffuse en direct l'office monastique des laudes, en direct de l'église Saint-Gervais-Saint-Protais (Paris 4e), avec les Fraternités Monastiques de Jérusalem."

I attended the Evening Song (Vepres) at 5.30 PM prior to the Vigil Mass on a Saturday in Notre Dame in 2013, which was very nice.

Not all Latin Masses are Gregorian chant Masses. Some are just the followers of Lefarve(sp) who refuse to give up the old Latin Mass. We attended one this September that RS book said was Gregorian chant, but it was, in fact, the Mass I attended as a child. It was an all in Latin and the choir were not much of a choir. A real disappointment for us.

ciaran - this might be the place.
I was thrown off by the fact that this church is on the Right Bank, not the Left.