Here are various .tex templates, sources or examples:

• Basic Tex Template for Gregorio Scores (you know, the basic one that you edited and now don't have a copy to reference any more….)
• Completorium (by Boris Maire)
• Template for missal (by Boris Maire)
• Generic template (by Elie Roux)
• Bilingual booklet
• Traditional completorium, intended to be easily adapted to every gregorian chant booklet
• GregorioSampler