(If this were a movie, this would be the description) ;)

It's Big trouble in little Korea as Seunghyun, the eldest son of the Choi family, is expected to take his fathers place as head of the syndicate. Based in a large city in the USA, the Choi family has had firm control on their neighbourhood for generations. But while he loves his family, Seunghyun doesn't feel he's up for the job, and is more interested in living a normal life.
When he finds Ji Yong and his friends on his doorstep, begging him to accept them as members of the syndicate, he does what he always does, but things seem different somehow. And before he knows it, and before he can prepare himself, the syndicate is being threatened from both the in- and outside, and if Seunghyun doesn't stand up to it, nobody else will.

TLDR:
Gangster AU taking place in a Koreatown in America.
TOP is the son of a Korean mob boss and he has to take his father's place soon.
GD wants to be a part of the syndicate and immediately hits it off with TOP.
GD/TOP slash + drama, guns, hissy fits, and kisses in dark alleys.