Paragraph 134 Under the heading of Mass with a congregation of the 2002 GIRM reads as follow (highlights mine)
134 At the ambo, the priest opens the book and, with his hands joined, says: The Lord be with you, with the people responding, And also with you. Then he says: A reading from the Holy Gospel…, making the sign of the cross with his thumb on the book and on his forehead, mouth and breast,** which everyone else does as well.** The people make their acclamation: Glory to You, O Lord. The priest then incenses the book, if incense is used. Then he proclaims the gospel reading and at the end makes the acclamation: The Word of the Lord, to which all respond, Praise to you, Lord Jesus Christ. The priest kisses the book, saying inaudibly:Through the words of the gospel may our sins be washed away.
In the 1975 GIRM under the heading Mass with a congregation
- At the lectern the priest opens the book and says: The Lord be with you. Then he says: A reading from. . ., making the sign of the cross with his thumb on the book and on his forehead, mouth, and breast. If incense is used, he then incenses the book. After the acclamation of the people, he proclaims the gospel and at the end kisses the book, saying softly: May the words of the gospel wipe away our sins. After the reading the people make the acclamation customary to the region.
I think they added it to the 2002 GIRM so that people didn’t question it when the congregation did it with the priest and I also think that the rubrics for the congregation weren’t included int he 1975 GIRM as the rubrics were intended to be shown to the congregation by the example of the deacon or MC (lead altar boy).