Q1. As is clear in the next paragraph, there is the option of taking the psalm and response from GR or GS. The GR may be the same as the lectionary, but the GS psalm will very likely be different. So the answer must be 'the actual particular psalm'. This is reinforced by the explanation given starting at However.
Q2. They are saying an alternative metrical translation is OK, if approved. The word 'paraphrase' is loaded, and does not really apply when what we have is a Hebrew original of which our understanding is coloured by successive Greek and Latin and English translations.
Q3. I am sure that there is no such MUST, which would in any case be superseded by GIRM.

In England&Wales there is no explicit permission for metrical translations, but that does not stop them being used. It is very common indeed at funerals to sing the Scottish Presbyterian metrical version of The Lord is my Shepherd.

pfreese, what is the source of your information that the GIA paraphrased/metered psalm settings (and counterparts from other publishers) were never intended to be sung as the responsorial psalm? GIA originally published many of the psalm settings that found their way into Gather hymnals as "Psalms for the Church Year" (a series). I quote from the Introduction of Volume Three of "Psalms for the Church Year":

In this volume, we have attempted to provide more settings for use with the Sunday lectionary.... The most obvious use of these psalms is in the context of the Liturgy of the Word.

Far from "many places never got the memo," I think rather that the paraphrased psalm settings were intended by their composers and heavily promoted by publishers as suitable replacements for the responsorial psalm in the lectionary, and no bishop that I'm aware of has ever disputed that nor prohibited their use in that way even though it's pretty easy to argue that they shouldn't be used because we shouldn't paraphrase nor revise an approved liturgical text nor the Word of God as officially authorized for use at Mass. Those paraphrased psalm settings change the authorized translation of the Word of God, which should be universally recognized as a big no-no for the Liturgy of the Word.

I once heard David Haas, Marty Haugen, and Fr. Michael Joncas talk about how their paraphrased psalm settings were, at least initially, intended to substitute the propers of the mass, as well as use during the Liturgy of the Hours, not necessary the Lectionary. With regards to the preface of Gather III, I took that to mean the expanded lectionary psalms by Michel Guimont (compared to GCII), which are in the actual Lectionary readings section of the hymnal while those by Haugen&co are listed in the LOTH section (just my impression, feel free to correct me if I’m wrong).

The GIRM’s talk about metered psalms is unfortunately not as clear as it should be, since conservatively speaking, that could refer to either psalm settings that are musically metered, or textually metered, which are two completely different things; one is a matter of musical style while the other completely changes the biblical text. Even within GIA they don’t seem to agree, where the Gelineau psalms are referred to being meterical (musically but not lyrically, since it uses the Grail) as well as that of Haugen&co’s Psalms of the Church year (which is both). IMHO, the “spirit of the GIRM” seems to favor the former but not the latter, given its permissiveness for qualified styles of music alongside chant, and its specificity regarding approved biblical translations for liturgical use (and the arduous process which that entails). It’s also worth noting that just because a collection of psalm-hymns received ecclesiastical approval doesn’t necessarily mean it was for the Lectionary, which in the case of Psalms of the Church Year is almost moot now that Haugen and Tony Alonso have come out with the Lyric Psalter (which uses the Revised Grail). I’ll still stand by what I said earlier that the proper lectionary text should be given the utmost precedence, and that to do otherwise would require a possibly overly-liberal reading of the GIRM.

Forum member Fr. @ronkrisman (who formerly served as executive director to the Bishops' Committee on the Liturgy) wrote a few years ago here that when the Gather hymnal was approved for publication, it means that the censors judged it to be free of doctrinal errors. OTOH, it doesn't make Gather an official liturgical book on par with the Graduale Simplex, for example.

So if someone suggests substituting Gather's psalm paraphrases in place of the psalm texts in approved liturgical books, it doesn't seem justified.

On the other hand, it might be helpful to sing one of them occasionally at a suitable time, to console the nostalgic who miss them.

That last point probably needs to be clarified: there is a distinction between approval for publication and approval for liturgical use.

The approved-once, approved-everywhere practice of USCCB's committee on divine worship refers to the latter.

If I understand Fr. Krisman's comments correctly, the Gather book got approval for publication, but not the other type of approval.

Yes, the USCCB hasn't really fulfilled LA on that point. And while STTL doesn't have legal force (it was never submitted to the Holy See to make it official), it does describe how USCCB is treating the matter.

As for the particular example discussed above, most parishes using the Gather hymnal have editions older than 2007. An approval for publication granted in the 1980s doesn't necessarily imply that the bishop was making a judgment on liturgical suitability.

I use the WLP psalms in the Let Us Celebrate missalette which are lectionary psalms. There's a ringer thrown in occasionally, but the problem is music not text. Still better than those Haugen/Haas psalms in the hymnal.

Yes, you're right, clear as mud. It's difficult to identify anywhere for a clear 'line in the sand'. Some bishops might support you, but USCCB won't guard your back.