You can email Ben Bloomfield and ask if the code isn't hiding somewhere in the Github repository, because I presume that you're referring to his propers tool.

Otherwise, I'd just typeset with Gregorio directly (which you can do via Overleaf if you don't wish to install a TeX distribution locally); you should be able to upload a font if it's not available in TeXLive (that is, accessible in the format \usepackage{<fontname>} — I mention this as you see this in the LaTeX tutorial on the Gregorio Project website.