Perhaps a musical expert can tell us where the dividing line is between the authentic and the imitation:
Moreover the removal of pieces that display later, neo-gregorian imitations, particularly in the feasts of the Saints, has made it possible that only Gregorian melodies be retained, although it is always permitted that those who prefer, may retain and sing those neo-gregorian melodies. None of these is completely expunged from the Graduale Romanum. Indeed, for some that have been accepted into universal use (e.g., in the Solemnities of the Most Sacred Heart of Jesus, of Christ the King; of the Immaculate Conception of the B.V.M.), no substitution has been made. In the place of others, however, selected chants from the authentic repertory have been placed, bearing the same texts, when possible.
Lastly, care was taken to more suitably arrange the authentic Gregorian repertory, purged of inauthentic melodies, so as to avoid repetitions of their texts as much as possible, and let other pieces of the best form, that only occur once a year, assume their places. Hence in every way effort was made whereby the Communions were enriched, gathering into those chants everything that was not strictly proper to some Saint, and hence can be taken up for all the Saints of the same order. Furthermore, the commons have been enriched with various chants, derived from the Proper of Seasons, which were rarely used. The rubrics present a more ample permission to draw from the newly arranged Commons, so that pastoral needs can also be more generously satisfied.
So there’s a good deal of flexibility in this. A core repertoire of “authentic” Gregorian music is identified, but anyone wishing to use newer music is permitted to sing it. There seemed to be an efort to ensure good pieces weren’t sidelined because the celebrations to which they were attached were less prominent liturgically. What do those folks fully familiar with the Roman Gradual have to say on these points? Have they worked for you?
Thanks to Richard Chonak who translated the second edition (1988) quoted here.