Go-Back-N Protocol not efficient?
The Go-Back-N protocol is often presented for didactic purposes as a first, maybe crude attempt at tackling loss of packets or of order in connection-oriented communications, without obeying the constrains of Stop-and-Wait protocols, also introduced for didactic purposes.  It is no wonder it appears inefficient -- it probably was never meant to be so.  Selective Repeat can be considered as a performance improvement in terms of retransmissions over Go-Back-N, and in practice TCP itself relies on a hybrid solution of the two.