Wrong again. The propers were not “re-written” after the council. You cannot re-write the Bible and the vast majority of the propers are from scripture, both psalms and other verses. The chants are the same, the melodies are the same, but some of them have been moved around. As I write this, I am sitting in front of both the 1974 Graduale Romanum (post-Conciliar) and a 1926 edition of the Graduale Romanum,
Let’s look at the propers for the 26th Sunday in Ordinary Time (post-Council) and 20th Sunday after Pentecost (pre-Council).
Introit: Omnia quae fecisti (Dan. 3, 31, 29, 30, 43, 42, v. ps 118)
Gradual: Oculi omnium (Ps. 144 15 v. 16)
Alleluia; Paratum cor meum (Ps. 107, 2)
Offertory: Super alumina Babylonis (Ps. 136, 1)
Communion antiphon: Memento verbi tui (Ps. 118, 49, 50)
The texts and melodies are exactly the same.
It’s pretty much the same throughout. As I mentioned, some adaptations were needed for Year B because of the readings from St. Mark, but often appropriate verses from the corresponding synoptic gospel verse can be used.
Another example, week III in Ordinary Time corresponds to Week III after Epiphany in the old Gradual but there are variants for years A and B.
In. year C: Adorate Deum (Ps. 96, 7, 8 and 1)
In. year A and B Dominus secus mare (Mt. 4, 19, Ps. 18); pre-council it was used for the Vigil of St. Andrew, Apostle; in Year A the text from Matthew is read about Jesus leaving for Galilee after the arrest of John the Baptist. In Year B, the corresponding text from Mark is read, therefore it is deemed appropriate to use the Introit which is a verse from Matthew’s text.
Gradual: Timebunt gentes (Ps. 101, 16, v. 17)
Alleluia: Dominus regnavit (Ps. 96, 1)
Off.: Dextera Domini (Ps. 117, 16, 17)
Communion: Mirabantur omnes (Luke 4, 22)
Again the melodies are the same. The same also applies to the great traditional antiphons for the seasons.
Vatican II was not the great train wreck some traditionalists make it out to be. I strongly urge you to study what actually is in the pre- and post-Conciliar liturgies instead of relying on hearsay from prejudiced interlocutors. Since the Propers are mostly scriptural, some moving around was needed to ensure that texts were appropriate to the readings. But again, the texts and melodies are the same. There have been very few new Gregorian chant composed for the Mass since the Council, but quite a few composed in the 20th century, for example for the feast of Christ the King.