Well Bach, Vivaldi, Telemann, etc are not exactly Gregorian chant. That's baroque. And Rachmaninoff borders on modern with his slight atonality.
Gregorian chant is pretty good if you like it, but it isn't classified as "classical" music. I'd suggest you try Santo Domingo de Silos. You might reconsider when you listen to a piece like this:
[youtube]ela-Z7HqqS8[/youtube]
And this is one of their best pieces, but I couldn't find the full piece for free:
http://listen.artistdirect.com/nad/window/media/page/0,,193585-557782,00.html
The name of the piece is Alleluia, Beatus Vir Qui Suffert (Modo I).
By the way, I am an atheist, but I love this music for the sake of music. Try yoga or other forms of meditation. It hardly gets any better or more peaceful than this; all the recordings were done in a remote church in Spain. This is not exactly a commercial recording since these particular monks don't like the commercial world. In fact, they have stopped recording. There are some other Benedictine monk chants, but I think these are the only worthy ones because they are genuine and sincere.