In the mid ninth century a monk of the monastry of St. Jean de Réôme wrote a manuscript in which he tried to apply ancient Greek music theory to the practise of Gregorian chant in his day. His name: Aurélien or Aurelianus; the name of the manuscript: Musica Disciplina. Here you find a presentation I made on the manuscript. The full manuscript can be found here. In the post Consulted literature you can find a full reference of the sources used.