GDK is an acronym meaning 'Gangster Disciples Killer'. This acronym is used by gang members to make aware of their intension to kill Gangster Disciples gang members. Any gang whom are enemies of the GDs can use the term, though it's mostly used by Black Disciples gang members, since they are one of the main rivals of the Gangster Disciples. There many other terms similar to GDK such as 'BDK' and 'EBK'. Gang members are known for adding a 'K' at the end of their rivals gang's name to mean they will kill them any chance they get.
GDK has gained popularity lately due to the rise of drill and trap music from Chicago. Many rap artists who gang members have been using this word in their songs. Ballout, a member of the rap group 'Glory Boyz Entertainment(GBE)' and members of the Black Disciples gang, used it in one of his song titled "My Set", in which he says "We BD, GDK on my f*cking set - Lil n*ggas everywhere and they holdin Techs".