The penultimate threats in Dragon Ball Z are the brutal androids, who were designed with the sole purpose of killing Goku and the other Z Fighters. Creating them and sending them on this task was none other than Android 20, a.k.a. the Red Ribbon Army's Dr. Gero. The good doctor was turned into the show's final numbered android to complete his revenge, making the seemingly frail old man into a much bigger threat.
This, of course, raises a lot of questions in terms of how the Gero actually became an android, why he did it and why he chose to become the type of android that he was. The show has answers for almost all of this, making what seems like a haphazard decision suddenly a lot more reasonable.
Who Was Android 20?
Dr. Gero was a member of the Red Ribbon Army, creating all of their advanced, numbered androids. One of these humanoid robots, the Frankenstein's Monster-esque Android 8, would eventually befriend a young Goku. Goku and his allies eventually defeated the Red Ribbon Army, with the boy being instrumental in ending their quest for world domination.
Gero had apparently always dreamed of using androids to replace the human race, using the resources of the Red Ribbon Army to do so. After their defeat at the hands of Goku, however, Gero went into hiding, hoping to study the young Saiyan further in order to build something to finally beat him. Building several all-new androids, numbered 16 to 19, these killers were primed to eradicate Goku and the Z Fighters. Android 16 was even based on Gero's deceased son, who was killed in battle. This quest for revenge would even see Gero himself dirty his hands on the battlefield.
How and Why Did Gero Become Android 20?
The most obvious reason why Dr. Gero became an android was to truly take revenge on Goku for his destruction of the Red Ribbon Army. Being able to join the fray of the fight would make the revenge even more personal. Even when Goku was a child, he still likely posed a threat to Gero. This was made even greater with Gero's age, as the old man would have been no danger to Goku whatsoever in his human form.
Thus, Gero had Android 19 implant his brain inside of an android body, becoming Android 20. He was far less organic than 17 and 18, but his still having a human brain in many ways made him arguably a cyborg and not a true android. This form was an energy-draining type instead of an infinite-powered model, the latter of which was stronger. Still, an energy-absorbing model has distinct advantages of its own. It would have allowed Gero to drain the energy of the Z Fighters, weakening them while becoming far stronger himself. Afterward, he would supposedly have the strength to defeat Goku once and for all. The androids would even succeed in this quest in the timeline of Future Trunks, but not so in the main universe.
Unfortunately for Dr. Gero, things didn't quite go as he had planned, and his android body was decapitated with one kick by Android 17. He would later also be easily beaten again by collaborator Dr. Myuu in the now non-canon Dragon Ball GT. Thus, while revenge is definitely a dish best served cold, it unfortunately for Gero was never eaten at all.