One of the real merits of the recording by the Choir of Christ Church Cathedral, Oxford, is the fact that the trebles are boys, singing this work in which the treble part soars high above the lower parts. The the festal Mass setting itself is notable for the cantus firmus being placed in the means (altos), rather than in the tenor. Here is an excellent article from Saturday Chorale.

The original is notated in with one flat, with final invariably D-major. For some reason, in an otherwise excellent performance, the Tallis Scholars recorded this work a whole tone higher, presumably because of their laser sopranos, for whom high B's are no problem, or possibly because of the low D's in the bass part. When I sang this work with Zephyrus many years ago, we sang it at notated pitch, and the two (and subsequently three) of us singing bass had no problems with the low D's. One of the high points was singing it at the Cathedral of St. Matthew the Apostle in Washington, D.C.

I had engraved the Gloria some time ago and published it at CPDL, and I had also engraved the Credo but didn't publish it. The entire Missa Gloria tibi trinitas is now on my "front burner" for engraving a complete edition, including a revision of the Gloria. For what it's worth, I'm attaching the Gloria as engraved back in 2002, with minor revisions and subsequently published at CPDL in 2006.

CHG -
Please correct the title of your conversation: an 'r' makes all the difference in the world!