It's what you hear in churches, all of the psalms and other christian songs are done the Gregorian way.
But Gregorian music isn't just for the religious people, and not all of the songs need to have christian lyrics.
An awesome formation, mostly German, who call themselves: Gregorian These guys are great at covering modern (pop)songs in the oldschool way. With their great voices they manage to give most songs an epic touch.
But see for yourself, here are a few examples of covers they did:
Just search YouTube, these guys have done a lot of covers,
or just go here: http://www.discogs.com/artist/Gregorian for their CD's and such.