Do you know of any composers (preferably modern) who utilize the style of polyphonic gregorian chant in their music? Musical Pieces?

Gregorian Chant Composers

The answers by others all steer you toward the path of other composers who may interest you, but I have found that many young people who like one piece by a particular composer will not seek out similar works by the same composer. If you haven't explored the entire body of works of Part, may I suggest that you do so. If you go to amazon.com and find a choral recital with music by Part, you may find similar music on the discs by other composers that will open your aural world.

See if you can find anything sung by the choral group Roomful of Teeth. A few nice works by Carolyn Shaw, among other things - and very interesting what can be done by using words like instruments, or using NO words. Also find the Q2 channel of WQXR in NY (they have an app of their own)- you will hear some amazing things, every single day. And yes - we also love Schnittke and Part - and a long list of others.

Try the composer John Tavener who died in 2013. His music all sounds a bit the same to me, but he has legions of dedicated fans.

I was in an orchestra which played a modal composition by Ernest Bloch.

I don 't remember the title, though.