The problem is that "pre-Gregorian" is also "pre-notational".
There are some recordings of what little Greek and Roman music there is, but interpretation is extremely conjectural

There's an interesting recording from about 20 years ago, "Chant Wars", which explores Gallican vs. Old Roman chant. Spotify here, for a listen. Ensemble Organum and Capella Romana may also have some relevant material.

That said, I question your premise. Chant is the living music of the Church, with an established performance practice (Solesmes). You don't want to give the impression that it's a museum piece.

There were western chant traditions that predated the Gregorian one, like Ambrosian, Gallican, and Old Roman. Of these, if I'm not much mistaken, the Ambrosian was the best preserved, as the Gallican and Old Roman traditions were sort of fused and replaced by the Gregorian while the Ambrosian more or less locally persevered. So one avenue for you might be to look into the oldest Ambrosian chants and compare them with Gregorian ones, perhaps even of the same text.

You're not going to get recordings of straight-up authentic historical performance, because nobody is certain exactly how this music, Ambrosian or Gregorian, sounded 1000+ years ago (and it's safe to say there was not one single, precise "correct" way of singing it throughout all those times and places).

I think I understand a little better now. Yes, as far as I know, modal psalmody (with intonation, reciting tone, and termination as you say) did precede the composition of more musically complex antiphons and such, and was a tradition carried on from Temple worship (among others). In passing, the Ambrosian Credo I'm familiar with is very recitative in this way.

So for purposes of illustration you might present a Gregorian and/or Ambrosian psalm tone or two as a very rough approximation of what much of the chant may have sounded like in the early-Early Church. They probably began composing/repurposing chant-like hymns immediately though.

You would want to add caveats. In the early centuries the psalmist might, when possible, have elaborated extemporaneously on the psalm tones ("performance practice"). And while we have the Gregorian and Ambrosian tones (and some of the Ambrosian ones may be earlier) we don't have the psalm tones of the Early Church (which may not even have come from the diatonic genus in all cases), or don't know that we do. So it would just be a very general idea of chant in the Early Church.

Very open to any corrections from others. Generally the history here is complex and opaque, so you're going to be both speculating and oversimplifying when you tell a neat story of its coming-to-be.

That said, I question your premise. Chant is the living music of the Church, with an established performance practice (Solesmes). You don't want to give the impression that it's a museum piece.

Jeffrey, I like your point about performance practice. :)

But what does it even mean? Are there no non-Solesmes performance practices that are "established" for Gregorian chant? Accentualists, semiologists, mensuralists, and equalists would disagree with your assessment. Is the chanting of the Sistine Choir, which doesn't follow the Solesmes method for Papal Masses, somehow non-representative of the living music of the Church? No, I think the situation is quite the opposite. The ones who most give the impression of chant as a museum piece are the ones clinging the most tenaciously to outdated scholarship, frozen since the late 1950s, usually under the pretext of aesthetic preference masquerading as obedience, authenticity, and/or purity. Like it or not, the last ecumenical council called for renewed scholarship, a return to the sources, and a more critical edition of the chant books published since St. Pius X. How many are taking the call seriously and doing their part?

Performance practice flows from a musical text. Even in performance practices where the text is not sacrosanct, the text is still the most important evidence of what actually went on. The approved musical chant text of the Church is a Solesmes text, though without the Solesmes rhythmic signs. The Church wisely does not dictate how this text is to be interpreted. But 99% of people singing chant in the liturgy are doing so from a fully-Solesmes text (LU/LB or similar digital editions) and are singing more-or-less in that tradition, even if it's often "Folk Solesmes".

I am in favor of scholarship, in general. But the job of the Church is not scholarship; it's the worship of Almighty God. If the Church were to decide that scholarship suited that end, that would be great. But the Church has not cared about chant in that way since 1964; indeed, it would rather that it disappeared or be left to the musicologists. Looking through issues of the Caecilia ca. 1960, one sees passionate discussion of minutiae, as the meteor bore down on the Vatican. I'm not saying there's no value in pursuing a platonic ideal of any given chant. But it's more important that chant be sung everywhere by all, in the received tradition (young as it may be).

I know I'm responding to a couple of people simultaneously, but speaking as one who is neither an academic nor a musicologist, I don't consider singing from the oldest sources to be fiddling, tinkering, tampering, or whatever other derogatory terminology one may wish to use, nor do I think of it as the pursuit of a platonic ideal. First and foremost, it is the proclamation of a sacred text handed down from the Fathers of the Church, as handed down by them. The official edition was a rushed project, with an estimated 50 years' worth of work compressed into five, and the Holy See acknowledged as early as 1906 (!) that changes to it would be needed in the future. There are some chants that should be known and sung everywhere by all, but I don't think that's the Church's desire for most of the chant repertory, which is more in the domain of trained singers and clergy or religious with a canonical obligation to chant the Divine Office. I see absolutely no reason why it should be considered desirable for every Catholic everywhere to sing graduals and offertories according to the Solesmes method, which you call "the received tradition."

I need to preface what I'm about to write by saying that I intend absolutely no disrespect toward the great Dom Gueranger, and I don't mean to derail this thread, but my impression from what I've read of his biography is that he and his founding confreres were secular priests who learned monasticism from books, not through a living tradition, which didn't exist in France. Is this an accurate assessment or not? If so, the whole Solesmes enterprise has to be viewed as a sort of educated reconstruction (I'll leave it to others to debate how educated reconstruction differs from academic fiddling) rather than tradition. As for the chant, the Solesmes method is neither traditional, nor Tridentine, nor an organic development, nor can it honestly be viewed through the lens of a hermeneutic of continuity, yet certain communities within the Church (and some without!) act as though the cantors were counting nuanced groups of twos and threes—and deliberately assigning arsis or thesis to each group—in pre-Revolutionary France, which is ludicrous.