GA OR SK "go ahead or stop keying" [2002, “Calling & Answering”, in Using Your Superprint PRO80 LVD, 1st edition, Ultratec, page 20:
When either of the two people is ready to end the conversation, this is done by typing at the end of a sentence—“SK OR GA” (stop key or go ahead). [1974 December, Leo Dicker, “Suggested Procedures for Using Telecommunication Devices”, in Jess M. Smith, editor, The Deaf American, volume 27, number 4, page 9: