That's a good question! It's what gregorian.ly does by default. I tried commenting it out, it doesn't seem to have changed anything. I've posted some files in case that helps.

I may have found the culprit. I commented out the \remove "Bar_engraver" and ended up with bars after every note. Fixed that by commenting out the "barAlways = ##t" line. Got the bars I wanted, but it looks like the barAlways was what allowed me to avoid using manual line breaks - I think LilyPond only does automatic line breaks at barlines. Dilemmas, dilemmas. :) I'll see if I can find a workaround via editing the division markers in gregorio.ly.

gregorian.ly is only necessary for engraving square notes in Lilypond. I think an example in the manual suggests using it for gregorian modern notation scores, but it is not necessary, and actually messes up the spacing of notes. When in modern notation, the only thing gregorian.ly gives you is the gregorian bar lines, which can easily be added without gregorian.ly by predefining the following:

divisioMinima = {
\once \override BreathingSign.stencil = #ly:breathing-sign::divisio-minima
\once \override BreathingSign.Y-offset = #2
\breathe
}
divisioMaior = {
\once \override BreathingSign.stencil = #ly:breathing-sign::divisio-maior
\once \override BreathingSign.Y-offset = #0
\breathe
}
divisioMaxima = {
\once \override BreathingSign.stencil = #ly:breathing-sign::divisio-maxima
\once \override BreathingSign.Y-offset = #0
\breathe
}
finalis = {
\once \override BreathingSign.stencil = #ly:breathing-sign::finalis
\once \override BreathingSign.Y-offset = #0
\breathe
}

I usually put a blank bar (\bar "") after the gregorian bar line, otherwise, the next note seems to crowd it.

The "Lauda anima" pdf above has some of the strange stretches between 3 note groups that are caused by gregorian.ly, and which also appear on the NOH github files. Those were part of an early foray into lilypond. They sort of replicate the look of NOH, but are extremely clumsy code. I have decided that I prefer something like the attached.

To avoid unwanted bars in chant-like (unmetered) acclamations, I've used two methods.

Either :
(1) use \cadenzaOn to disable automatic bars; add all desired bars manually;

or:
(2) Count up the beats in each phrase and set its meter explicitly (e.g., to 14/8 or 9/8 or whatever the length of the phrase dictates) and let the system put the bar at the end of the phrase. Since I'm also suppressing time signatures, the oddly numbered meters don't display.

cantorconvert,
That's very helpful to know about defining your own Gregorian bar lines without gregorian.ly. Thank you!

chonak and ryand,
I have done that (option 2) in other notation software as well, and it certainly has its benefits. Personally I prefer the on-the-fly method in Lilypond (I usually copy \bar "" to the clipboard and this makes it very fast), and the "pre-counting" method in the point-and-click notation software.

Of course, in spite of this, we still have the dilemma of automating mid-phrase line breaks in chant without "baralways." So far it seems the consensus is that this still needs to be done manually by guessing and checking with the compiled score.

Perhaps we can get Aristotle in this discussion. I will send him a message.

I use Frescobaldi and find it very helpful. I do all my testing with it.