The GIRM is part of a liturgical book, the Roman Missal. The Code of Canon Law has in canon 846 “The liturgical books, approved by the competent authority, are to be faithfully followed in the celebration of the sacraments.”
Before a priest is ordained as a bishop he takes an Oath of Fidelity, which includes: “I shall foster the common discipline of the whole Church, and I shall insist on the observance of all ecclesiastical laws, especially those which are contained in the Code of Canon Law.”
The bishop does not have the authority to change what the GIRM n. 138 says about the person announcing the intentions “facing the people”. The things an individual diocesan bishop can decide are described in GIRM n. 387: “… It is to him that in this Instruction is entrusted the regulating of the discipline of concelebration (cf. above, nos. 202, 374) and the establishing of norms regarding the function of serving the priest at the altar (cf. above, no. 107), the distribution of Holy Communion under both kinds (cf. above, no. 283), and the construction and ordering of churches (cf. above, no. 291). …”.
2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from romanrite.com/girm.html
The Code of Canon Law: New revised English Translation, HarperCollins Liturgical, 1997, ISBN 000599375X.
John Huels, The Pastoral Companion, Franciscan Press, 1995, ISBN 0-8199-0968-8, page 379…