Future Dr. Gero, or Future Android 20, is the alternate timeline counterpart of Dr. Gero.
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(Android 20) 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.