Leo XII wasn’t saying that Catholics must not read Bibles translated by Protestants, even if they were good translations. He was saying that only the authority of the Church can declare if a translation is good or not.
If a translation receives the imprimatur, Catholics can read it for instruction in the faith, because the Church has declared it to be accurate. It doesn’t matter whether the translator was protestant, pagan, or atheist. (Leo XII was in particular against the Protestant Bible Societies, because they were formed to disseminate the text of the Bible apart from the Body of Christ, the Church. The Holy Father’s objection was that the two are inseparable, but the Bible Societies were actively working to undermine that.)
The GNB has been approved (in the editions with the deuterocanonical books included) as a good translation (taking into account its dynamic equivalence approach, and simplified language), but it is not approved for liturgical use (the holy liturgy is not an appropriate venue for a simplified version of God’s Word).
That means it cannot be used at Mass*, but there is no problem with studying it to know God’s Word, especially for those who find the more literal, literary, translations (like the Douay-Rheims or New Jerusalem Bible) difficult to understand.
** Actually it may be the case that individual bishops can authorize it for use in the Mass in particular cases, but I’m not sure about how that works–that’s way above my pay grade!]*