Selective acknowledgement (SACK) effectively indicates which sequence numbers are missing when there's packet loss. It's optional, but afaik, used by nearly everything on today's internet.
What would a NACK add? TCP can already send an ACK for the last successful sequence number, telling the sender to retransmit packets after that sequence number. Due to latency and large window sizes, it's far more efficient to just resend all the data than NACK individual packets.
Perhaps GP considers a NACK more semantically easier to understand on an intuitive level, instead of the implicit NACK with "I know you sent 8, but we're really at 4 so resume from that". But I do agree that it's more efficient for the remote host to just immediately establish "4 was the last point we are ok at".
