The trope is the musical equivalent of the gloss, particularly interesting to music historians. The trope was an addition of words or music (or both) to a preexisting Gregorian chant melody in the Mass. Usually, the trope was bound in meaning to the text to which it is added; by the 1200s, trope writing's popularity had nearly come to an end. Unfortunately, changing the original chant was not looked kindly upon by the Catholic Church, as nearly all tropes were removed by the Council of Trent in the mid 1500s.
So, if the Church didn't favor this new addition to chants in the Mass, why did it take nearly 400 years to even decide to remove existing tropes?