Gregorian chant is just singing, there's no instrumental backing. Also the melodies used in Gregorian chant are also a lot simpler than those used later in Carmina Burana.
Founder of Jaco society

[22:08:23] <Confusius> I wish I was a bassist
[22:08:26] <Confusius> you fuckers look cool

Want to know how to play bass in jazz? Read this.
Yeah, it's just a beautiful Classical Choral piece. I LOVE this piece, but it's not Gregorian Chant. Gregorian Chant was used mostly back in the day when the church considered instrumental music evil... therefore it was just vocal.
Fury- Breathe in deep, and cleanse away our sins
And we'll pray that there's no God
To punish us, and make a fuss

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