I'll admit that GATHER is loaded with Protestant hymns, but that's because the Protestants wrote good hymns.
If we did as you suggest, all we'd be left with is tripe like "Daily, Daily sing to Mary."
No thanks.
In my view, nearly every thing in OCP hymn books *is* tripe. Marty Haugan and David Haas have done more damage to Catholic liturgy than...well, I am not sure. The Huns?
If I have to sit through one more rendition of "On Eagle's Wings..." God help me. Even as a penance that's a cruel one.
Adoremus makes a decent hymn book. And it does include some Protestant hymns, by the way - the best ones, at any rate (I concede there are good ones).