(Please note, this has no real Batman characters)
Name of Gang (If it has one): Gladius Domini 
Gang leader: Gladium Rex (Nightmare)
Notable gang members: Satis Deridere (Karajan), Noctiu Agus (Ursa)
Headquarters: A cave system outside of Gotham
Enemies (If any): None
Allies (If any): None
History: 
An immigrant who's parents where Italian Mobsters, Gladium Rex was raised with crime. Multiple times having near death experiences because of his parents mistakes when he was a child. He moved to America at the age of 18 to escape the life of crime but it kept coming back to him. Multiple times he ended up in Jail for thefts and disturbances but no murder. 

Gladium Rex decided that there was no way to escape his criminal ways, so he started to create an image for himself. With the persona of Nightmare. He gathered some friends he met on the streets and formed a gang. As Nightmare, he wears a black mask with red flames painted on. He carries a sword with him at all times and a gun for jobs he needs to pull off. Gladium Rex has never murdered, but Nightmare has.