Open Source Liber Usualis - Chants and Propers for the Latin Mass
The file LiberUsualis.mgabc contains most of the text of the Liber. The code will be adapted for Gregorio 4. The primary source of the text is the Divinum officium project at https://github.com/DivinumOfficium/divinum-officium.
The Garamond font in Ubuntu is found in the package texlive-fonts-extra