In Future Trunks' timeline
In some flashbacks of Future Trunks' timeline, Dr. Gero is seen as a Human after the Red Ribbon Army was defeated by Future Goku, having not made himself into an android. Later in this timeline, Future Dr. Gero (in his Android 20 form) is destroyed sometime after he activates Android 18 and Android 17; Future Android 18 distracts Dr. Gero by knocking over some equipment and 17 cuts his head off. As he warns them about Cell absorbing them, 18 blows up his head. When he is killed, the Super Computer he made to finish Future Cell is already working, so it continues and eventually finishes its work.
In Cell's timeline
In Cell's timeline (shown in a flashback in episode "His Name Is Cell"), Future Dr. Gero is shown as a human working on Cell. Later in Cell's timeline (shown in episodes "Ghosts from Tomorrow" and "The Horror Won't End"), Future Dr. Gero is shown to be Android 20 just as his present timeline counterpart.
During Imperfect Cell's flashback to Piccolo, it is explained how Dr. Gero had discovered a way to fuse together the cells from the strongest warriors to walk Earth (Goku, Piccolo, Vegeta, Frieza, and King Cold) and create a supreme fighting entity. After realizing that his project would take too long to be completed before his time, he decided to turn attention to other matters and leave his supercomputer from the lab to complete Cell.
During Super Perfect Cell's flashback, Gero is shown killed by Future Android 17 and Future Android 18. He is killed as he finishes off the settings for the supercomputer to begin working on completing Cell.