As always, I note the kind contribution of Richard Chonak who translated the Latin original of the second edition (1988). Today, some guidance on how to use the repertoire in the Roman Gradual for the reformed liturgy:
19. Since a great variety of readings has been introduced to the Missale Romanum, while the chants of the Mass from received tradition cannot be changed, the assignment of chants is
being brought into accord with the various readings, according to the three-year cycle (A, B, C) of the Lectionary established for Sundays.
Also, for ferial days, the chants of the preceding Sunday are repeated, and they are being brought into accord with the readings assigned to each day of the special seasons of Advent, Lent, and Easter, as well as with the first readings during Ordinary Time, according to the two-year cycle (I, II).
If a chant appears that is joined in a more or less strict relation to some readings, it ought to be brought with them if the readings happen to be transferred.
20. Any exceptions that may be added to the Proper of Seasons are represented in this Order, after each basic assignment, by the following written abbreviations: A, B, C for Sundays, solemnities and certain feasts; I and II with the numbers of weekdays (Saturday is indicated with the number 7) for ferial days in Ordinary Time; numbers of weekdays alone for the ferias of other seasons. The abbreviations written in this way are placed in another part of this book, where all the exceptions are linked together, no. 136-141.
21. The chief norm which this Order of Chant for the Mass follows is that it strives to observe the Missale Romanum as much as possible in its ordering. For this reason some of the assignments of chants are being transferred or altered.
Those who work closely with the proper chants and the Roman Rite may be able to relate if this adaptation has been more or less successful in harmonizing with the Lectionary readings and seasonal considerations. Anyone?