I think the main reason Gentlemen received mixed reception was because it wasn't as big of a hit as Gangnam Style. And well... of course it would never be as big of a hit! Gangnam Style is literally a one-in-a-lifetime milestone, no successor would receive 2 billion views on YouTube.
And while this song is mediocre at best, I get what Psy is trying to do. He's trying to sing a song that most Americans would recognize in rap standards and well... it's kinda similar.