In the case of The Geese Book Gradual, the term "Gradual" refers to a particular type of book. A Gradual book is a compendium of all the musical pieces in a mass, omitting the spoken parts. These books were often very large to allow several choir members to read the music from a standing position.
page from a Gradual by the Italian Illuminator Antonio de Monza, circa late 1400s-early 1500s, via the Getty
*If my freshman year Humanities professor (who once gave me a less than favorable grade on a paper because I cited wikipedia in my bibliography) could read this, I'm sure he'd do a facepalm. Sorry, Prof!**