Communion antiphon (specifically the one proper to the Mass)

Most here would take that one to be: in Latin from the GR, or in English a setting of a translation of that one to a comparable tune. But I cannot see the GR antiphons as being intended for anyone but a trained singer, well rehearsed.
GIRM lists plenty of alternatives, all of which are allowable, suited to various styles of performance and various degrees of participation. For maximum simplicity there is the GS option of always using Ps 34/33V Benedicam Domino with antiphon Gustate quam suavis, or the same in English for maximal participation.
Marc Cerisier : this article discusses extensivly why the antiphons do not always match.

Why 'they' didn't put translations of the GR's antiphons in the missal instead of the ones that they made up from whole cloth is 'anyone's guess'. Whatever their inevitably sad reason, it displays yet more of Roman liturgical ineptitude.
As it stands now, people are choosing what is near at hand in the missal (in full knowledge that the missal antiphons are not meant to be sung), and the Gregorian repertory is going begging. The GR propers (all of them!) in translations that lend themselves to musical expression are what should be in the missal.

Each member of the congregation has a copy of the Graduale Triplex, and has successfully been taught to sight read its contents. At Communion all come in procession singing from their books, which they lay on the altar rails while receiving. They return to their places and continue to stand and sing until the distribution of communion is completed. Surely Kathy, if the world were like that there would be no need for entrance hymns to familiar tunes.

Anyone who can sight read a Sanctus can sight read this.

Especially after hearing the schola sing it a couple of times, during my reception of Communion.

What you are describing is Responsorial Psalmody, as in The Responsorial Psalm. The Responsory (NOT Antiphon) should be sung first by a cantor and then repeated by the people. The Responsory may be sung after every verse, after every several verses, as you please. The verses may be sung by a cantor or a choir. It would be best to print the Responsory, if not the entire psalm, in a service folder or mass leaflet. Good luck with this. I hope it works out for you and your people.

James - we do it every week at all of our masses at the cathedral in Sioux Falls, SD. The exception being one Sunday morning Mass when choirs are in session, where we have a chant schola that sings the Graduale proper for the entire procession.

I wrestled with the practicality question as well, which led me to compose my own settings for each sunday. The antiphon, in modern notation, is printed in the liturgy guide. It is sung once by cantor or choir, repeated by all, and then alternates with psalm verses. Other than mine and Andy Motyka's I am not aware of any comprehensive english language congregational communion antiphons (I do not consider the SEP a congregational resource). I suppose there are some simple psalm-tone resources that might be considered congregational, though.

I'm in the end stages of putting all of mine online for free (I've been navigating the copyright questions with multiple offices all summer), and others can judge the worth and usefulness of those for themselves. In general, though, I think if we are serious about the congregation singing proper antiphons, we church musicians have a whole lot of composing to do! It's not enough to presume that simple chant resources will work well or at all with typical congregations. The more antiphon collections that are composed and made available, the more options we will have and the more likely we will be to sift out the best ones.

All of that said, the idea that the congregation should sing the proper is revolutionary in the extreme, and possibly just plain crazy. The proper repertoire was never intended for the congregation historically, and it cannot necessarily be magically modified to be so simply because in 1967 some people thought that would be neat! I think you have to consider that the church is talking about seminary and religious congregations as well as regular parishes, when advocating for congregational propers. Also remember that Musicam Sacram paragraph 33 talks about participation in the propers - possibly by simple refrains or other methods. The idea that lay parishioners will be able to sing the propers themselves (i.e. the complete Graduale propers, in english or latin) does not seem to me to be possible. Communion propers, if they are of a suitable refrain length (i.e. often shortened even from what is in the Missal) and in modern notation, do work in my parish. But I'm not sure the congregational proper project can go much farther than that.

I would like to stress that the Graduale simplex wasn't provided with the intent to accommodate congregational singing, but in the first place to meet the needs of choirs for whom the chants of the Graduale Romanum were too difficult.

That doesn't take away the fact that the simpler antiphons from the Graduale simplex – and their English adaptations – are quite suitable for congregational singing.

For the last three Sundays, I used the simple antiphon "Taste how good the Lord is" (GS 460, in Dutch) as a congregational refrain during communion, with the cantor singing verses from Psalm 34 (33) and the people responding. The congregation did pick it up quite easily, even as they processed to receive communion.

I think it is important that, if you want the congregation to join in the singing of the communion antiphon,
- it has to start already while the priests communicates; that way there's time for the people to pick up the antiphon while they're still in the pews and can read the text (and music) from their worship aids;
- it has to be simple and short, so that they can continue singing it with ease without the help of a leaflet or misalette while processing to receive communion;
- the text and melody should foster a prayerful preparation for receiving communion; for example, the antiphon could emphasize the close relationship between the two tables of the Word and the Eucharist (by quoting the Gospel); or it could make people more aware of the Eucharist as nutrimentum caritatis, the 'food of charity'; etc.

GS Praenotanda:

GS(1967) #14a [=GS(1975) #18a]
Cantor antiphonas inchoat, atque versus psalmorum proponit, populo respondente.

V. Persons needed for the singing of the chants of the Simple Gradual
14. The guiding principle is set forth in the Constitution on the Liturgy: "Each one, minister or layperson, who has an office to perform, should do all, but only, those parts which pertain to that office by the nature of the rite and the principles of liturgy" (SC #28) Therefore, in the case of the Simple Gradual :
a. Antiphons are intoned by a cantor, who also leads the psalms, with the congregation responding. Psalms may also be sung by the choir.
b. Antiphons and the response to the psalms between the readings should be sung by the entire congregation. .... (DOL 533)

You might consider looking into the Ignatius Pew Missal by Fr. Samuel Weber. We are on our second year of using this in our Ordinary Form parish. The entrance antiphon, psalm, and communion antiphon are in English, and the notation is printed in the missals. We had not included propers in our liturgy until using this resource (this was a VERY foreign and practically FEARED concept). I integrated them gradually over the course of a year (starting with the Communion Antiphon and singing it only by myself at the time). Now, as a congregation, we sing the Entrance antiphon, psalm, and communion antiphon together. The chant is simple and repetitive, which at first bothered me because I thought it wouldn't be musically interesting. Now, however, as I see the confidence growing in the congregation, I am thankful that it is not more difficult, as that can lead to discouragement. I do sing it in a responsorial style so that the tune can be better grasped, and by about halfway through, the church is filled with singing (to my shock and delight). Hope this helps! http://www.pewmissal.com/brand_new/