Hi JC, I live to the west of Ganhyeon in Yeoju. There's lot's of camping there, but very packed and overcrowded at times. The crag is right across from a big car camping area. But there are a few places near the end where it's not quite as crowded.
It's a very busy crag so you either need to camp out or get there early to really get some climbing in before the crowds. If not, then just go into with some patience. A lot of Koreans climb with clubs and in big groups, so depending on the day, there may be a lot of climbs occupied for a while. So, just go with the flow.
Let me know if you need a partner sometime, I live close buy and can usually make it over pretty easily. Also, checkout this facebook page if you haven't found it yet https://www.facebook.com/groups/185477421498238/
...more people seem to use facebook now, so it's good to know.