The expression is based on a video with the same name performed by the Korean artist PSY. The pop artist did all choreography (fake 'horse-riding' etc), music, lyrics etc himself rendering the end result an Internet laughing hit and is by many categorized as one of those rare things too bad in itself to mock by making adaptations or parody of, hence making it an Internet phenomenon. The song in the video is a catchy pop song, helping the phenomenon to spread. The expression has nothing to do with the Gagnam District of Seoul, South Korea, but just with the questionable result of the pop artists attempt to do everything in regards to his video.
"Did you see Johnnys first commercial at advertising school? It was SO gagnam style... Hope he gets better."
"I know, I know, I should have seen my limitations and not tried to both Build it and Configure it myself, but at least it was pretty Gagnamn Style.. Right?"