The practice of troping the Kyrie was especially common across the duchies of northern and northwestern France in Normandy and Brittany, which then spread to England as part of the Use of Sarum. The tropes could be unique at each abbey or cathedral, but some spread, hence their place in the Graduale.
Here is an arrangement of the troped Kyrie Orbis Factor. It is based on one from the Graduel de Aliénor de Bretagne, a medieval gradual from the abbey of Fontevraud, an important monastic house in the ancient county of Anjou. This gradual contains greats example of chant in the 13th and 14th centuries. I am not an expert on this, and we know we cannot replicate precisely the style of chant in a particular period or even place. There also might be alterations to the text and vocal style made that I am not aware of. That being said, it seems to me that this is at least partially successful. It's beautiful and hauntingly so.
For whatever reason the Church decided on simply singing Kyrie Eleison and Christe Eleison in the ninefold sequence without any additions, and I can't even imagine singing this at a liturgy using the printed Missale Romanum in any year after 1474 (basically today's Mass in the Extraordinary Form). It is so very different, from an age gone by, but one that I feel we Catholics need to be grateful.
I would like to learn more about the medieval liturgy (liturgies, really, since there were so many uses and customs) and its musical form. Chant, of course. It's interesting to hear the different forms of Western plainchant, so different from our usual Gregorian chants. It seems after Trent the desire was for a liturgy formed top-down and one that was mostly standardized, and even though the medieval mentality cannot be recovered, perhaps its acceptance of multiple laws of prayer is something for us moderns to examine.