The New World is probably one of the best Korean gangster films I've seen so far and I've seen a ton of them so if you watched Gangnam 1970 and liked it watch The New World.
Both have Lee Min Ho as the male lead and both have action in it.
They both have a similar tone to it, but Gangnam 1970 doesn't have a lot of comedy and doesn't have a lot of romance.
Both show the gritty side of The Republic of South Korea as it developed. Though, The King takes place 20 years later when transitioning from a dictatorship to a democracy took place. Corruption plagued both eras.