I'm working on adding to the Nova Organi Harmonia project on GitHub, but rather than start from scratch, I wrote some Python code to convert Gregorio code to LilyPond notation, output it to a table (CSV) which I open in a spreadsheet editor, add the ATB notes. This is then fed through another script which outputs a LilyPond file.

I've done it this way to avoid having to figure out the note lengths myself. Remember the MOD format they used on the Amiga? Same idea. :) I'm posting it here in case anyone else finds it useful.

https://github.com/ahinkley/gabc-to-ly