This is kind of an unusual question for several reasons.
First, most all modern harmonic music is still influenced by polyphony, which means “at the same time”; but, second, when it comes to more traditional-minded Catholics, they usually like non-polyphonic music.
Gregorian Chant is mostly non-polyphonic. It’s monophonic, which means one note at a time, and it commonly called “plainchant”.
So - if you are looking at ancient music, and yet want to listen to polyphonic music - you aren’t really speaking about Gregorian chants.
Look to music by Palestrina and Josquin de Prez and their contemporaries - also bearing in mind there were secular artists, too.
Polyphony continued up to the common practice era through Bach, Mozart and Beethoven and still exists today. So there’s really no stopping point to it.
Just make sure to discern between the styles of plainchant, polyphony, and harmony .
Also, you might want to include “counterpoint” in your searches, as counterpoint is the theory upon which polyphony is really built (as opposed to something strictly harmonic per se).