One repeats words or phrases in their proper location (something approved by the church by her approval of polyphony), and the other takes texts from completely different places. A "responsorial" gloria is far different than polyphony. One is repetition of texts, the other is the reordering of texts.
Just to check, where is this crucial dichotomy of "repetition" vs. "reordering" found in any authoritative documents of the Church?
So the Credo of Mozart's Coronation Mass (KV 317), which ends, "... et vitam venturi saeculi, amen (x20). Credo in unum Deum, amen, amen," would rule it out for sacred use in your mind. Okay.
Yes, that would rule it out. Not the approved text. Old ≠ always correct.
GIRM 68: The Creed is to be sung or said by the Priest together with the people on Sundays and solemnities. It may be said also at particular celebrations of a more solemn character.
If it is sung, it is intoned by the Priest or, if appropriate, by a cantor or by the choir. It is then sung either by everybody together or by the people alternating with the choir.
GIRM 53: [...] It [the Gloria] is intoned by the Priest or, if appropriate, by a cantor or by the choir; but it is sung either by everyone together, or by the people alternately with the choir, or by the choir alone. If not sung, it is to be recited either by everybody together or by two choirs responding to each other.
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