Why does johnny compare the gallant southern men to dally

As the above answer states, Johnny links Dally to the Southern gentlemen in terms of gallantry and courage. Dally may not share the Southern gentlemen's refinement of manners but Johnny, at least, is convinced that he is equally brave. Dally does show this, in a way, when he faces up to his own death at the end of the book; in fact he instigates his death at the hands of the cops because he doesn't want to live anymore after losing Johnny.