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.
Mark Harmon studied sign language for his various televison roles with The Sign Company, which was run by owner Bill Pugin. Marlee Matlin, his co-star in Reasonable Doubt coached him in ASL. He also sporadically uses sign language in NCIS.