For the OF, the Mass for Ash Wednesday is used on weekdays of Lent in the 1975 GS. That said, the 1975 GS Mass of 5th Sunday in Lent is actually the 1967 GS Mass of Passiontide, so it would make sense to me to use that Mass for Spy Wednesday. Just make sure to use at least 5 verses for the Responsorial Psalm. I believe there's only 1 reading before the Gospel on Spy Wednesday, so you have a choice of Responsorial Psalm I, II, the Acclamation (with verses from the previous Responsorial Psalm that weren't used), or the Tract.

@ClemensRomanus, is the older GS available on-line?

It seems to me that the (ordinary form) GS has little to say about ferial masses, except for that one Ash Wednesday rubric. And since (apart from the responsorial psalm) the OF rubrics leave so much freedom to the choice of chants (cf. GIRM #367) I'd read it as permissive or encouraging -- "These songs can _well_ be chosen for any weekday in Lent" -- rather than restrictive.

As for the responsorial psalms, I think they are the glory of the GS, the best reason to keep it. Unfortunately they are so unusual, so far from common usage, that in my experience priests and people and choirs are reluctant to choose them. Not sung the usual antiphonal ish way, but more like responsories. Not the same texts as the Lectionary. And longer, as you mention above. Weirds people out. You're lucky to have a priest who's keen on them.

(We are using By Flowing Waters, which is the GS in English, sort of. But we have to sing the responsorial psalms from the lectionary.)

In theory yes, but in fact and practice not at all.

The brief responsories in the Office, which is what I mean, are like this
* the cantor starts with a verse
* the response follows immediately, completing the thought ("locutio integra" as it says in the GS praenotanda, or "emerges from the text" as Paul Ford says)
* each verse is responded: the exchange is relatively rapid
* verse and response are relatively short and simple
* often there are two response melodies, and/or two verse tones

By contrast, the usual Responsorial Psalm:
* the responses are relatively long (eg a whole Psalm verse)
* the cantor's verses are often sung two at a time
* the response rarely "emerges from the text"
* the cantor invariably begins with the response, which is repeated as a first "response" before the first verse
* the response melodies are often self-contained musical phrases

The result is more like antiphonal singing, where the "antiphon" can stand alone, and is resumed after some verses of a psalm. Of course the verses are from the cantor alone, usually (though we've probably all seen it done other ways, and the rubric allows several other ways).

As for the "responsorial" Gradual, as found in the Graduale Romanum, well, historically it's said that this is or was a responsorial form, but
* There is only one "verse" and the "response" is not usually resumed after it, only sung once before
* The "response" generally is a whole verse.
* The melodies are long and florid and glorious, and not at all fitted for the back and forth of a responsorium brevium.

So whatever the theory, in practice the GS responsorial psalms are not like these other things.

I suppose the 1968 ICEL translation is still normative, isn't it. Heh! Because the Holy See hasn’t recognized any revision of it.

@ClemensRomanus, would you know of any online resource providing the English translation of the praenotanda and rubrics of the 2007 GS? Our young schola would like to seriously consider learning the GS chants since we often get invitations to sing at such short notice. The simpler chants are quicker to learn. Though, we still cherish the GR as our first choice.

is the older GS available on-line?

This claims to be downloadable, though I don't know how to get it! (advice welcome)
Apart from the reassignment needed by the reform of the calendar, are there any differences between the editions? (And of course the rubric changes for the OF)

Regarding your suggestion that the Mass chants for the 5th Sunday of Lent can be used for Spy Wednesday — I'll probably go this route, since we've rehearsed this chant set before. Though, will such a choice not be a violation of the above-quoted rubric?

The rubric itself doesn't prescribe that the Ash Wednesday chants are to be used on the weekdays of Lent.

Hi cantus resumi possunt in omnibus feriis temporis Quadragesimae.

Possunt gives a possibility. These chants may be used throughout the Lenten weekdays. I agree with Andrew that rubric is permissive rather than restrictive. The chants of the 5th Sunday of Lent are a good choice for Wednesday of Holy Week, which doesn't violate the rubric.