It is currently only available for Finale. I've pleaded with the creator to port it to Dorico, and I'm hoping that will happen in a future version (although I suspect we'll still be waiting a while... Dorico needs to open up more of their scripting architecture to 3rd-party developers).

I know at least one colleague who uses it but he's warned me it is somewhat finicky in the end. You also have to like using Finale (I downloaded a finale trial just to try out the plugin and I decided I hated using finale so much that it wasn't worth it even for the plugin).

Gregorio is indeed time consuming and daunting at first, but there are things like gregobase which has almost every chant under the sun already rendered in GABC which you can then simply edit (ie- you don't have to program it from scratch). You might still get further this way.

I haven't yet tried to do chant in Medieval 2, for reasons others have stated. I may try it soon for chant incipits and alternatim settings, which I've generally done with black breves and note placement suggesting ligatures. It's almost a program-within-a-program, with the learning curve that suggests.

I do a lot of incipits for early music editions. I've got Bravura and Aruvarb (the allegedly Finale-ready version of Bravura) and was hoping I could take advantage of the expanded SMuFL character set. But they really don't play well with Finale, esp. in contexts where one has both early and standard notation. The major problem is that actual notes only have one stem direction. Finale already has acceptable diamonds for minims and semiminims (less acceptable for semibreves), but Finale attaches notes to stems by their sides. It's not a big deal to move the stem to the lower/upper point of the diamond, but then the stems poke into the open heads of the white diamonds. One cam move the stems vertically, but apparently it's the stem end that defines pitch, because the notes no longer line up properly on the staff.

Still, I've had good luck with the following:
Clefs: in the template I'm using, I've replaced clefs in the clef tool with 16th-c. clefs. The only problem here is that, if one has a key signature, the flat in the bass and alto clefs appears outside the staff. The cure for this is in the nonstandard key signature tool, and doesn't work for me half the time, so I do without the actual key signature and place a 16thc flat as a staff expression.
Mensurations signs can be placed as text (pretty much any of it can be) in Staff View (so they move with the staff)
Better: import your symbols as Staff Expressions, so that they connect to a beat, then move them where they need to go. I do this for rests, accidentals and ligatures.

There's a JW Lua script for creating longas and maximas, but the stems all point down.