Argh argh argh argh argh no.
First off, the “Communio” is the DEFAULT. I grant you that in US Catholicism, EF or OF, most people have never ever heard a Communio chant, but it’s still the default option, the first option, the option that is supposed to be normal.
What is a Communio chant?
It is a designated liturgical text, assigned to a specific Sunday in the liturgical cycle. There is an antiphon with a designated chant melody, and then there are designated psalm verses, which may include the entire length of the psalm if needed. You can sing it in Latin or English. If you want to be really different, you sing a different melodic setting of the Communio chant, in whatever style is appropriate. And if you’re really feeling wild, you sing a metrical or song version of the antiphon and psalm verses. Woohoo! Livin’ crazy!
Way after all that, you have the option to sing a hymn at Communion instead, or in addition to it, after the Communio.
The GIRM is somewhat mis-emphasizing the importance of the Communion chant or hymn. It is supposed to primarily continue the lectionary theme, allowing us both to worship God and be educated by His Word about these things. At the same time, it does allow us to thank God for the wonder of the Eucharist, whether or not one happens to be receiving said Eucharist; but very often, the designated chants for Communion are not explicitly about Communion. Heck, most of the time. The Mass is a whole, and the lectionary texts of the day are a set, not Lego bricks of all different shapes and sizes.
The Communio is actually a lot more like a “closing song.”
(Because there is no designated liturgical text for singing at the end of Mass. After the “Missa est,” since we don’t have the Last Gospel in the OF, we’re done. If the EF has a liturgical closing song, it’s technically the way they used to chant the Last Gospel reading: “In the beginning was the Word…” And since laypeople were encouraged to stick around and pray after Mass, you really didn’t need a song processional anyway since the people weren’t going anywhere right away.)
So yeah… they’re not exactly giving you good info, in the GIRM. This is why there are many other documents on church music to consult. The Communio is holy, sure, because it’s the Word of God and it’s in the lectionary. J. Random Hymn is only holy as a sort of Nutrasweet version of the Communio.
Anyway: here’s the list of designated readings (or chantings, really, since Mass readings are really all intended to be sung – that’s another default thing that we miss out on):
Introit antiphon and psalm verses (usually replaced by the “opening song”)
Psalm or gradual
Alleluia, or alleluia and sequence
Antiphon for the offering of the gifts, in the EF; nothing in the OF
And I don’t hate hymns. I love hymns. But I hate missing out on all this spiritual food from the Psalms illuminating the rest of the readings, and it’s ludicrous that music directors don’t usually even try to hit the themes of the antiphons when hymns are supposed to be replacements for them. Why? Because we’ve totally forgotten them over the last 150 years, in favor of hymns.