It plays similar to Maple Story, a 2-D sidescroller where you mash the attack button on enemy faces and jump over obstacles while traveling to the right. You have a choice between three different classes: The Ranger, The Mage, and The Warrior. It seems like most of their play styles are similar excluding special attacks that may be earned later. Currently this game is only available in Korea and there is no talk of it being translated for the states. Here are the videos of G’nGO from the beta. Check’em out!
Full Party Castle Dungeon:
In this video you can see that, yes, you can still run around in your underwear. There is an armor meter and after taking enough damage you will lose your armor until you can acquire more from the chests in the dungeon. Here he is starting the boss fight in his skivvies: