I just responded to someone about using TSPG at regular OF Masses. Here is a quote from a friend who helped with the creation of Divine Worship: The Missal:

"DW:TM follows the GR 1974 by and large for Sundays, Solemnities, and major Feasts, but obviously not for Trinitytide which follows the older usage, rather than the sliding scale for calculating Sundays in post-Pentecost Ordinary Time."

The melody edition includes a Table of Correspondence that shows the differences, which are mostly in the Introits and Communions. We have been using some of the Graduals, Alleluias, and Tracts at our OF Masses, and our Pastor seems to like the change from so many years of Responsorial Psalms - as do we musicians! As to the legitimacy of using them at OF Mass, at least in English with the current Lectionary (with between the Readings options) and the Sacramentary (with its paraphrases of the Antiphons), I figure it's largely up to the priests to allow it or not. I have thought about creating a Gradual, using as much of TSPG as possible, shifting Propers that exist to match the GR, and using the same text sources that my friend used to create whatever Propers are missing. But I haven't begun that yet.