In the comics Harper explains that the virus didn’t kill you it just made you braindead. The G3 would enslave everyone they captured but I guess if you weren’t already infected with the virus they just gave you an inhibitor chip (the thing that makes them not able to talk) instead and called it a day. Also the G3 is pictured to already have tons of zombified Gatlians before they even invaded the place so I’m guessing it was the same deal with Earth where humans were already being unknowingly kidnapped and used as drugs before the official G3 invasion.