Joker's Gang is a large group of career criminals operating in Gotham City, lead by the murderous Joker. The gang commit acts of extreme crime that almost always lead to large amounts of death and destruction, usually for the Joker's own amusement. Most, if not all, of them suffer from some form of mental illness; The kind of minds that the Joker attracts.
It's unknown how the Joker's gang came to be, but is known the clownish madman upset the order established by the Falcones and Maronis and would begin to rise in GC's underworld. Jonny Frost would join the gang sometime early on in the gang's creation and eventually become Joker's number one man in the syndicate. Presumably, the gang would topple all other villain's criminal empires, aiding Joker become the city's top crime lord while having encounters with Batman.
In present day, Joker is depressed his lover/partner is gone from his life for quite some time and has Frost look for her. By bribing some high-level officials, Frost learns Quinn is held in a maximum prison facility in the Louisiana swamps and informs his boss who uses his resources to find Harley and save his lover. Joker and his thugs travel across the country to get a prison official, Griggs, to help them in Joker's plan. They learn of Task Force X and nano-bombs and later attack Van Criss Laboratories to force the inventor of the nano-bombs to help them save Harley.
They hijack a military helicopter in Midway City in an effort to save Harley but are shot down by the US Military, killing them all while Joker and Harley survive.
When Harley is returned to her cell in Belle Reve after she helps the Squad kill Enchantress and Incubus, with her little reward, Joker and another gang of thugs attack the complex, killing several guards and free her from her cell with Joker and Harley reuniting.