Gregorian Notation has been possible in Finale for many versions.

However, there are several “versions” of Gregorian Notation.

Which version are you asking about?

Gregorian Notation can be done without a special "module", and without paying extra for Finale.

However, you can buy extra, third party fonts for Gregorian Notation.

I do not think that anybody has created a list of Gregorian Notation symbols available with a standard Finale installation.

By The Way:

Finale comes with some finished Finale documents that demonstrate notation in various styles, including Gregorian Notation.

Have you considered downloading the 30-day demo version of Finale v25 (if you are running a 64-bit operating system) to see what it can do?

Gregorian Notation can be done without a special "module",

No, it can't but you can sort of fake a modern version of chant in Finale—it's still modern notation but it sorta' kinda' looks like something resembling chant. If singers have never seen plainsong, it's quicker than learning the real thing.

You cannot, however, write plainsong in Finale since the PPC. There was a special tool once (Midieval) but it's discontinued and was never updated to run on Intel Macs. You can't even buy the old version—I tried.

Here's the thread from last summer:

https://makemusic.zendesk.com/hc/en-us/community/posts/213612308-Gregorian-chant-notation

You'll find plenty of information (both plainsong and faking it in Finale), resources and links to a freeware chant notation app that relies on a decent knowledge of UNIX (not for the faint of heart).

Buried in that page are links to links... If you are really looking to write plainsong instead of modern notation with a 'chant look' …

Here are the old and new sites for The Gregorio Project:

http://home.gna.org/gregorio/

http://gregorio-project.github.io/

The other is using a text editor plus the Caecilia font. This is one of many solutions found in this overview.

http://forum.musicasacra.com/forum/discussion/7895/software-how-do-you-write-your-gregorian-chant/p1

I really like what Fr. Matthew Spencer has done and may explore that a bit more.

Don't get me wrong. I am not putting down the use of modern notation or dressing up Finale to imitate some of the chant-like basics. If that's what you are looking for, go for it. It will certainly be easier than Gregorio.

I was raised on plainsong and chant having been taught in 2nd grade. It's easiest to write with a pen. Unfortunately, that option is no longer available to me. I was planning a chant project this summer but have pushed it back a year. Part of it is that it's impossible for me to use a pen anymore; the rest is that I'll have to teach a few people how to read plainsong. I may decide to just do it in modern notation instead and teach how to sing in spite of what is on the page.

Why don't we have Gregorian Chant Notation under Finale?

Most of all looking for it. 

Wow, old posts ... If this board was organized into threads, there could be a sticky with current info. Just sayin...

Anyway, back on topic. Since the above was written, Robert Piéchaud's Medieval 2 for Finale has been released. If you want true plainsong notation in Finale, this is how to get it. 

https://www.klemm-music.de/notation/medieval/en/index.php 

Otherwise, it is possible to get a chant 'look' in Finale without a special module per other posts on this board. It remains modern notation but sometimes, that's what users really want.

You can do a search for chant in the box at the upper right of this page and come up with many posts on this subject including this:

https://makemusic.zendesk.com/hc/en-us/community/posts/115004600234-chant-notation 

More info on Medieval 2 from the Finale blog:

https://www.finalemusic.com/blog/robert-piechaud-releases-medieval-2/ 

Thank you Mike for your informations!

The Medieval 2 for Finale seems very interesting!