No matter how many times Gibbs heard that question it still annoyed him, but that didn't mean that he blamed the person asking. It wasn't Elijah's fault that NCIS didn't get the credit that they deserved, so he kept his annoyance to himself while he mentally prepared his answer. He didn't want to make things too technical, but he also didn't want to come off as condescending either.
"Naval Criminal Investigation Services" he finally started. "We investigate the crimes that involve either the Navy or the Marines" he added, trying to explain it the best that he could.
And now he chuckled when he heard Elijah's response to his question. He didn't think it wise to admit that he sometimes slapped his people on the back of the head to get their attention. So, instead he simply said "Oh I get it".