The following antiphon (with versicle and collect) is said in honour of St Benedict, mindful of the promise made to St Gertrude the Great that whoever recalls that great saint's holy death will have his aid at the hour of death; I read somewhere that in former times it was sometimes said in monasteries after Compline, but I have been unable to find the chant for this antiphon.

Does anyone have the Gregorian chant for this, if it exists?

The closest I have found is a most beautiful recording of it sung in organum by Peter and Julian Kwasniewski, and composed by the latter.

Does the existence of this organum suggest a pre-existing chant, a cantus firmus as it were, or is the whole setting newly composed?

Here is the Latin:

Aña. Stans in Oratorio dilectus Domini Benedictus, Corpore et Sanguine dominico munitus, inter discipulorum manus imbecillia membra sustentans, erectis in cælum manibus, inter verba orationis spiritum efflavit; qui per viam stratam palliis et innumeris coruscam lampadibus, cælum ascendere visus est.

V. Gloriosus apparuisti in conspectu Domini.
R. Propterea decorem induit te Dominus.

Oremus.

Oratio.
Deus, qui pretiosam mortem sanctissimi Patris nostri Benedicti tot, tantisque privilegiis decorasti; concede quæsumus nobis: ut cujus memoriam recolimus, ejus in obitu nostro beata præsentia ab hostium muniamur insidiis. Per Christum Dominum nostrum. R. Amen.