Gerald Jackson was a young African-American man who at the beginning of Season 1 was Donald Mallard's medical assistant.
As such, Gerald regularly aided Ducky whenever it came to performing an autopsy on a body in the morgue or if they were called out to a crime scene.
For six or seven months, he worked with Ducky on many cases, helping the team out whenever it was necessary but was forced to temporarily resign from his job after the events of Bête Noire (episode), when terrorist Ari Haswari whose real identity was unknown to the team at the time infiltrated the NCIS morgue, taking Ducky, Kate, and Gerald hostage in exchange for the body and belongings of a terrorist that was in the NCIS morgue. He eventually shot Gerald in the shoulder joint in revenge for Ducky lying to him before managing to escape although he left a wave of trauma in his wake.
Presumbly, the psychological trauma and the shot of being shot proved too much for Gerald to handle and he went to rehab for treatment, temporarily reassigning from his Assistant Medical Examiner position. Numerous unnamed people aided Ducky until James Palmer took on the role on a permanent basis.
Gerald made a re-appearance during the events of Kill Ari Part 1, when he was seen buying pills for his shoulder. Ari broke into his car and took him hostage again, intending to force Ducky into a meet as he hoped to explain to Ducky that he had not killed Caitlin Todd despite the fact that he had pulled the trigger that had ended her life.
Ducky later agreed to the exchange without the knowledge of Leroy Jethro Gibbs and attended the meeting, replacing Gerald while forcing Gibbs on a chase across the city to search for Ducky and Gerald.
Upon being released, Gerald fled to NCIS HQ and notified the team of the situation. It was also revealed that he still maintained a good friendship but Abby had nearly forgotten about him altogether.
As such, Gerald's current whereabouts and status are unknown although it's presumed that he has either returned to rehab or has moved elsewhere.
As a result, Palmer who once held the role on a temporary basis has permanently replaced him as Medical Examiner Assistant in both the morgue and the field.
Unlike his replacement, Jimmy Palmer who seems eager to learn all he can from Ducky, Gerald was the exact opposite, displaying no interest of any kind in Ducky's stories, preferring to spend as much time as possible on his MP3 player or working in general, tuning Ducky's ramblings out. Despite this, he always performed the necessary duties and helped out as well.
He had something of a good friendship with the rest of the team but he very rarely spoke on other cases.