As one of the major antagonists of Dragon Ball Z's Android Saga, Dr. Gero seemed to have little to fear. However, the one thing he does seemingly "fear" is Android 16. In an expansion to his character exclusive to the Funimation dub of Dragon Ball Z Kai, Dr. Gero insists that Androids 17 and 18 don't wake 16, saying that his programming is incomplete and he could destroy the world. However, given that he's the one who created the Androids and knows how to take them out, his concern is rather odd.
Instead of fearing Android 16 himself, Dr. Gero's reasoning appears to be he doesn't want to lose him. Android 16 is a recreation of Dr. Gero's dead son, Gebo, who died at some point before the show. Gebo was a Red Ribbon Army soldier who operated under the codename "Gold" before he was shot and killed by an enemy. Dr. Gero, who was a Red Ribbon scientist before the faction was destroyed by Goku as a child, designed Android 16 to look like Gebo and programmed him to kill Goku as revenge for destroying the Red Ribbon Army.
Despite being a robotic killing machine, Android 16 is a nature-loving gentle giant who only wants to protect others. Because of his gentle disposition, Dr. Gero declared him a failure and never activated him. However, it appears that his reasoning is deeper than that. Because 16 looks just like the son he lost, Dr. Gero probably couldn't cope with the thought of losing him a second time. He never wanted him to be harmed or destroyed, so he programmed him to love all life, with the exception of Goku. He also didn't want 16 to destroy the other Androids before they could accomplish their missions. However, Android 16 deems them to not be evil, and thus has no interest in destroying them. However, Dr. Gero's fears would be realized when Android 16 faced Imperfect Cell. Despite being almost equally as strong, Android 16 was killed by Cell during the Cell Games.
He's since shown up in other Dragon Ball media, usually lamenting that he doesn't really want to destroy Goku, despite his programming. Dr. Gero's hatred for Goku is what ultimately led to not only Android 16's destruction, but also his own. Had he not wanted to kill Goku, the Androids, including Cell, would never have been created and Future Trunks' world would never have been destroyed. In spite of viewing himself as immortal and above human limitations, Gero still held human-like emotions towards his son and the fear of death. When Android 18 suggested activating Android 16, it wasn't the destruction of the world Dr. Gero was afraid of. It also wasn't because his programming was incomplete, though it was a contributing factor. It was the thought of losing his son for a second time.
