The Six-Gum Gang members clockwise starting from the bottom: Runt, Lunk, Goof, Wilbur, and Dixie.
The Six-Gum Gang are a gang of cowboy-style robbers who work for the Delightful Children From Down The Lane, and Father. According to the credits, their names are Runt, Dixie, Wilbur, Lunk and Goof. Runt is the leader of the group, despite being the smallest and seemingly the youngest.
They made their first appearance in Operation: R.O.B.B.E.R.S., where they staged a hold-up on the morning school bus, stealing other kids homework and giving it to the DCFDTL so they can get extra credit for it. They were only shown stealing A+ homework. The Six-Gum Gang actually won in this episode, but their actions backfired on their boss(es) when Mrs. Thompson read Numbuh 4's terrible homework, and believing it to be their work, sent them to detention.
In Operation: F.O.U.N.T.A.I.N., Lunk was delightfulized and used by the Delightful Children from Down the Lane (possibly due to his failure in Operation: R.O.B.B.E.R.S.). He tortured and interrogated Leaky Leona to ascertain the location of the Fountain of Youth and was ordered by his brainwashers to watch for intruders as they went down into the catacombs under Gallagher Elementary School. Lunk was then distracted by a shiny hoop bracelet that had dropped from Leona's wrist and was immediately knocked out by Numbuh 5, who had been eavesdropping in the stall behind him and kicked the door into him, hitting his head through the wall. Lunk was somehow undelightfulized and returned to the six-gum gang.
They later appear in Operation: E.L.E.C.T.I.O.N.S., where they were appointed as the DCFDTL's personal Hall Monitors when the latter are dishonestly elected as school president(s). As such, they also controlled Gallagher's Permanent Detention facility and were ordered to keep Nigel incarcerated and under heavy surveillance, knowing the threat he posed to the DCFDTL's plans. After he broke out however, they were fired from the force and tossed into Permanent Detention along with their employers after the DCFDTL were removed from office (though this is not shown).
In Operation: S.A.F.E.T.Y., they are seen attending a "butt-busting" contest at the school playground and attempted to hold off the Safety Bots but failed.
They were last seen in Operation: I.N.T.E.R.V.I.E.W.S., trying to steal one of the Truck Driver's pianos, which Lunk accidentally crushed Runt with it, similar to what happened to Numbuh 4 in Operation: P.I.A.N.O.