We have yet to address the psalm and gospel acclamation. These considerations conclude Chapter IV of the GILM.
89. Among the chants between the readings, the psalm which follows the first reading is of great importance. As a rule the psalm to be used is the one assigned to the reading. But in the case of readings for the Common of Saints, ritual Masses, Masses for various needs and occasions, votive Masses, and Masses for the dead the choice is left up to the priest celebrating. He will base his choice on the principle of the pastoral benefit of those present.
But to make it easier for the people to join in the response to the psalm, the Order of Readings lists certain other texts of psalms and responses that have been chosen according to the various seasons or classes of Saints. Whenever the psalm is sung, these texts may replace the text corresponding to the reading. 
The priority for the psalm is for the people to sing it. To make that easier, a substitution may be made. My own sense is that any community that has been singing the psalm for several years is probably at the point where the psalm of the day could be sung. If, that is, psalmody is any kind of priority.
90. The chant between the second reading and the Gospel is either specified in each Mass and correlated with the Gospel or else it is left as a choice to be made from those in the series given for a liturgical season or one of the Commons.
91. During Lent one of the acclamations from those given in the Order of Readings may be used, depending on the occasion.  This acclamation precedes and follows the verse before the Gospel.
And you all knew there are common Gospel verses as well as assigned ones, right? Probably less need for special cases in this musical instance, given that the cantor or songleader or even the choir will render the verse.