Abby and Gibbs sign to each other often. We know that Abby knows sign language becuse her parents are deaf. But why does Gibbs know it. The only episode i saw anyone ask him he changed the subject quickly.
It's debatable that Harmon knows ASl from a communications degree, especially since he doesn't actually sign it in NCIS. Gibbs (Harmon) signs closer to SEE [Signing Exact English], or pidgin. My military boyfriend pointed out to me tonight that snipers in the Marines are sometimes taught pidgin so that they can communicate silently. I think it is an astute conclusion that canonically, Gibbs knew pidgin from his days as a sniper, and upon hiring Abby (or being hired on to work at NCIS, where Abby was working) he perhaps improved his SEE to solid proficiency.