Although Gibbs can easily return to NCIS, Harmon needs a compelling reason to return, and there may be no greater reason than for him to pay his respects to Ducky.
Both are founding cast members who helped launch the long-running police procedural when it debuted in 2003.
That being said, their friendship predates the events of the story. Ducky and Gibbs met in the 1980s, when both were still young men exploring their personal and professional futures.
This is also why Ducky should feature in the NCIS Gibbs prequel.
During Harmon’s departure episode, the two shared an emotional goodbye. Gibbs expressed his gratitude for Ducky’s companionship, emphasizing how solid their link is.
Given this, it’s tough to see him not returning to DC after learning of the death of his oldest friend.
Even if he does not take part in the actual case that the team will address in the tribute episode, Ducky’s appearance during the final rights would suffice.
Harmon issued a statement in the wake of McCallum’s death. It only makes logical that Gibbs would pay respect to his NCIS character.