Is there a style of music that incorporates Gregorian chant and hard rock/heavy metal?

It seems that some bands use it intermittently in a few songs but I cannot find any info on a band that uses it in every song.

The list i found includes the following bands:
Tristania
Sirenia
Therion
Enigma
Delerium
Nightwish?

I am familiar with nightwish, they do have a lot of of chior singing not sure if it is considered Gregorian though.

You may also want to check out Luca Turilli, and Rhapsody as well.