GIRM 61. ... The Responsorial Psalm should correspond to each reading and should usually be taken from the Lectionary. It is preferable for the Responsorial Psalm to be sung, at least as far as the people’s response is concerned. Hence the psalmist, or cantor of the Psalm, sings the Psalm verses at the ambo or another suitable place, ... [common psalms, in lectionary ≈ GS] ...
Instead of the Psalm assigned in the Lectionary, there may be sung either the Responsorial Gradual from the Graduale Romanum, or the Responsorial Psalm or the Alleluia Psalm from the Graduale Simplex, as described in these books.
Responsorial Gradual from the Graduale Romanum
By Flowing Waters (Paul F Ford) translates GS into chant in English. If I am correct in thinking that they are the same as the common psalms, the difference is just singing verses or stanzas, BFW congregations get twice as many responds.
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