I was reading this comment and saw that it was right: there is indeed a vast amount of chant transcribed in Gregorio's gabc format and no music typesetting software serious about supporting chant should ignore that.

Chant notation rendering in LilyPond isn't great, but I decided it was worth providing better support for the gabc input format anyway. For a long time there have been several different utilities capable of converting gabc to LilyPond code, but all of them require runtime environment of some programming language (and often some additional dependencies) - and AFAIK none of them is able to generate code to reproduce the score in LilyPond's square notation. So here comes lilygabc, a new LilyPond library translating and rendering gabc right in LilyPond, no external conversion program needed. It can generate both modern and square notation. Square notation support is still very incomplete, but small improvements are added almost daily.

I don't expect it to be tremendously useful - almost any other software capable of chant typesetting produces better looking results. But if for one reason or another there are gabc scores to render and it's LilyPond's task to deal with it, lilygabc can help. Otherwise it's just one more toy in the music typesetting toolbox, showcasing LilyPond's extensibility.