Here is a version of the Angelus formed by piecing together various chants, as an exercise in using Gregorio for the first time.
Here is the source.
Gregorio is a new package for setting Gregorian chant. As I write this in Jan 2009 it is only at version 0.9.2, and so one would expect many small bugs. I used it combined with gregoriotex as a backend.
My overall impression is very favourable: it produces output which seems good to me, and was much simpler to learn than Lilypond or Opustex. Line breaking is automatic and good in practice. I expect that it would be easy to integrate into a larger document, using TeX. Most ligatures are formed automatically, and so all you usually have to enter are the words and notes.
Some of the small bugs or limitations which remain are:
Another point which I do not like about it, clearly a design decision rather than a bug, is the use of letters to identify positions on the staff rather than (relative) pitches. Even the letters a-g only match the notes conventionally meant by them if the clef is c4 (C on top line). Fortunately the gabc is simple enough that one can easily write a preprocessor to generate it.
But these are minor issues; it seems to be the best free (liber) software for setting chant.