One could not dabble in this topic with noticing Charles Tournemire's l'Orgue Mystique, which uses the propers of each Sunday as the basis for organ composition.

https://www.youtube.com/watch?v=GIDJ2iTWFEI

The piece terminale for Pentecost.

This piece use the sequence for Pentecost (quoted at the beginning), the Veni Creator and the antiphon at Matins (used as a type of "glue" for the whole opus.)

Sadly, the pieces are not easy, as has been said countless times. But they remain the pre-eminent representation of organ music based on chant.

And I am biased as I am writing the man's biography.

Is this a duplicate thread? I just posted the following in another thread:

CHRIST LAG IN TODESBANDEN is an adaptation of a medieval chant used for "Victimae Paschali laudes"

CHRIST LAG...

And also Christ ist erstanden.
Not to mention Nun komm der Heiden heiland and scads of other German chorales. Adapting chant hymns into chorales for use in Lutheran liturgy was, as most here are likely aware, quite common. I don't know of another nationality in which this happened so pervasively - certainly not amongst the Calvinists, the Huguenots, and most others.

Too, there are numerous hymns in the Anglican repertory which are metred versions or loose quotations of chant tunes. The Hymnal 1940 and The English Hymnal, to name but two, are chock full of them. I'll be more specific tomorrow.

Chris didn't put any limits on the types of melodies or quotes, or whether they should be strictly liturgical, or orchestral, or other genres. This could be an endless thread.