Gregorian chants at Notre Dame

Yes:
https://www.classictic.com/en/cantigas__gregorian_chants_and_medieval_music_at_notre-dame_de_paris/33557/269774/?fin=c8

It would be better if you did, because this will be prime vacation time in Paris.

*The reduced ticket price is available for students and unemployed people with valid ID. *

The Arts are so important to the French that they give reduced-priced tickets to the unemployed so that they won't be denied the same opportunities as fully-employed people. This is beautiful beyond words.

-- Mike Beebe

I believe that the offer of reduced price or free tickets for the unemployed, requires some advance paperwork and is limited to French and EU citizens.

We went to an organ concert in the middle of June last year and did not need to buy tickets in advance. In fact, we got in and bought our tickets before our friend who had already purchased her ticket through the above-referenced site made it in (two lines). I can't imagine that Gregorian chants would be a high-demand event.

I attended a concert @ Notre Dame one evening last May London's Parliament Choir) and had to show my passport @ the security check. It was noted on the ticket itself but first time I've been asked for passport ID so head's up to check yours. While I'd been inside Notre Dame many times, to see it from the interior while the sun was setting was spectacular especially as the light faded behind the rose windows! The Gregorian Chants would be amazing!

I've never seen anyone check ID's for concerts in churches. There will probably be someone checking bags, though, so it's best to go a little early, especially if you do not have assigned seats,
The only time I've seen ID checks is for concerts in places like the Olympia, where there are often hucksters selling fake tickets on the sidewalk.