Careful: There is a treble version (SA only) out there.

Neumann Press has previously reprinted the 1941 edition (2 volumes, choir edition, spiral-bound), but that's out of print. There may be copies around.

A 1966 edition; probably better than nothing, but maybe limited?

http://www.giamusic.com/product_search.cfm?criteria=gregory+hymnal&search_button=search

You can also try Abe Books. They carry hundreds of old and rare books. I've purchased from them before without any problems. It's worth checking out.

Thank you everyone. I had no idea I would get such a big response!

You'll all be hearing from me soon. I hope to be able to purchase at least one copy from each one of you.

And yes, I was hoping something along the lines of the Neumann Press edition - which boggles the mind why it's gone out of print. (Yes, I know that Neumann has been acquired by TAN, but still . . .)

Maybe not out of print; I've placed several orders for it with GIA over years and they always have it (ppb and also a spiral version). For some reason they don't make it too easy to find on their website.

abridged? St Gregory Hymnal has been changed around many times. In fact I think I have several iterations dating back to the 1940s.
You can download the 1920 version here https://archive.org/details/TheStGregoryHymnalAndCatholicChoir/page/n1