The Finale plug-in you're thinking of is called Medieval. It is almost as expensive as the Finale software itself. I haven't used it, so I can't speak to its efficacy or its learning curve.

What I think most people here use for chant engraving is one of the Gregorio interfaces, either Adam Bartlett's (at Source and Summit), or Richard Chonak's. With these editors you can notate a chant and export it as a graphical file, then import that graphic into Finale. Both of these editors are free to use; you just have to learn the principals of using GABC (there's a helpful cheat-sheet at Chonak's website), and a long and thorough tutorial here (by Adam Wood, I believe).

@ LauraKaz - I am looking for four line notation and square notes. I do use Finale with stemless notes for modern notation.

I am using Apple Pages and the Caeciliae font, the GABC is good for quick things where you do not care too much about presentation and good use of the page.

I’ve heard the learning curve is *quite* high for medieval. That said, I’ve also petitioned the développer to port it to Dorico, and he’s apparently open to the idea, but Dorico needs some more support for scripting apparently.

Gamba, that’s one of the reasons I’ve bothered to have a go at GABC. It’s not terrible once you get the hang of it, and, as you said, there’s such a huge pool of data ready for the taking. My main pet peeve with GABC is the lack of control over the automatically inserted hyphenation. It’s very erratic.

I never got on with Medieval.
But with the current SMuFL-compliant version of Finale (27.3), it's unnecessary.
Chant in Finale is always going to be kludgy, and Gregorio produces better results. But for incidental chant (incipits, alternatim) I prefer to have the chant be an actual part of the file, rather than be a graphic imported from another program.

The general process:
Create a 4-line staff.
Create a clef using the glyphs from the SMuFL font set. You'll have to knock out a clef from the 18 available clefs, but it will only obtain for this piece unless you create a template (advised), and you probably won't be using guitar tab in this project.
Enter pitches for your first phrase, as quarters, planning how many you will need and giving the phrase the corresponding time sig, which you will hide. Note that you can create some neumes (torculus) by entering as chords and then use the Special Tools to move the noteheads around. Or just use the SMuFL multiple not forms.
Use Staff Styles to remove stems, except for virgas etc.
Utilities/change/notehead and select one of the SMuFL noteheads (Medieval and Renaissance planchant single-note forms)
An early and imperfect example may be found here: https://www.cpdl.org/wiki/images/2/29/Audi_benigne_Trent_92.pdf

FWIW (and since the OP asked this as a general open ended question and I’m known for making this reply) I use Dorico to great satisfaction for transcribing chant. Dorico allows you to work in open meter (no hiding time signatures or barlines!) so all you do is enter all the notes, adding tick and quarter bars only as you need them, then select all, use the jump bar to type “hide stems” and they all disappear in one go, et voilá. Done. It’s very quick.

@ Jeffrey Quick - I haven't gotten the 4 line staff to work yet (I did just update my version of finale, so maybe that will help) but your comment to "Use Staff Styles to remove stems" is a game changer. I use to remove stems by going to Special Tool: Custom Stems and creating a blank stem, and you can only apply this to one measure, one layer at a time. Ugh. It was so slow.

Rvisser-
4 line staff is easy IF you know where to look, but it's certainly not intuitive. If you go to the Score Manager and highlight your staff/instrument, in the lower right you will see "Staff [Standard 5-line]". That's a pulldown. Pull to "other"and it will give you the "staff setup" box. In the upper left of that, find "Staff lines", change it to "4" and hit OK. Note that this actually removes the bottom line of the staff, so soprano clef is no longer a thing. You'll want chanty clefs anyway, which means setting them up using the chant glyphs in SMuFL. It's harder than the 4 lines, but the manual instructions actually work, and once you have them set up you can save them as a template and not have to worry about them again.

Marc- thanks for the tip. I've only just begun to work with the JW plugins (I've had them/known about them but never really used them. JW Staff Polyphony does miracles for cut and paste.)

Re neumes: I'm editing some more from the Feroci-Matucci MS (by popular demand) and the chant there is written in black lozenges (and occasional diamonds). I've been entering neumes as chords and then using the Special Tools Notehead Position tool to switch notes to either side of the stem. But I've noticed that words align to the stem, not the leftmost note, so I'll just keep everything to the right of the stem and get better alignment. The notehead distances refer to the stem, but they don't at all have to touch the stem (which will be gone eventually anyway). Also, if you want a level torculus in lozenges, you can enter CDE, double-click the C and enable vertical positioning, and slide it up to D. Note that none of these techniques will work for you if you want chant that will play back correctly.

I still have not bought into Dorico nor have I upgraded to Sib 7… this forced me into a holding pattern and I’m using Musescore 4. For free software, it really is pretty good. However, I do not know if you could do any kind of chant notation.