People in Seoul and Incheon are really not into Pokemon Go the way HK people are. All the gym pokemon are beaten down, and you can easily win a gym in ten minutes.
In HK you'll need an hour, if you can do it at all, because even if you go out at some ridiculous time like 2 AM, someone will still feed and replace gym pokemon as you beat them. If a gym your color is under attack, you'll have a hard time trying to sneak in a replacement pokemon before someone else does.