I know I have a thing about title translations (OK, translation in general) but 'Gangster Boss' is surely one of the most stupid. What's wrong with 'The Big Chief', which is not only more literal but far more appropriate? This is adapted from O. Henry's story 'The Ransom of Red Chief'. Anyway, Antoine (Fernandel) and Paolo (Gino Cervi) work in a car wash but want to run their own: and the best way they can think of getting money is by kidnapping young Éric (Joël Papouf), the son of wealthy Alain Jumelin (Jean-Jacques Delbo).
The first problem with their scheme is that they're not built in the gangster mode, they're just big softies. And the second is that Eric is a handful, causing Antoine and Paolo endless trouble with his Red Indian games, and giving them sleepless nights. In the end they give up on the ransom and just hand Eric back. The trouble is that the Jumelin household have the same problems with Eric, and as penance the would-be kidnappers have to look after Eric every Sunday, which is not a prospect they relish. This, of course, is the burlesque side of Henri Verneuil.