Well, unlike JCS the text of the songs of GS is pretty unproblematic, so the only issue really is the suitability of the musical style for Mass.
JCS is both musically out-there and at least somewhat questionable theologically.
Now I do love the music of JCS and can listen to the lyrics without it affecting my faith, but would never think it suitable for Mass.
When I was in school, however, I remember ‘Prepare Ye’ being used in a prayer service (not Mass) held during Advent. In which context the simple exhortation to prepare for the Lord’s coming was quite appropriate. And I remember singing ‘Day by Day’, but am not sure whether it was in Mass or not.
I should add both were done with minimal instrumental accompaniment - so no tambourines or jazzy piano etc snd none of the handclapping either. Which makes a big difference.