The Gap of Chaos (better known as GOC) was originally one of God's fragments: which were destroyed and became the main transportation of most Kelimans (akin to a Chaos Gate).
When Veltus had created Chaos, the world of confusion, the Gods trained Dragons and created shields to protect Elgard. However,humans were so greedy, and with that, they started the war against Dragons.
When the war between humans and dragons had ended, dragons followed Avinor to leave for Ariastel. With no more dragons to protect Elgard, Veltus kept on attacking one of God's fragments. One day,he managed to destroy a shield, and a great many Kelimans entered Elgard under Veltus' control. That shield was then called Gap of Chaos by Veltus and the humans.
Upon entering LVL 60 (50 before the Priestess patch), the player is allowed to enter the Gap of Chaos for 30,000 Gold or 30 Cubics. The area of battle is selected by random, and here, Kelimans are much more stronger, faster, are way more corrupted than normal. Here,you are given a chance to get Unique Items: Items stronger than Legend Gear. However,the risk of the GOC being random is so risky: getting into a run with Relic Watchman is dangerous and you need to have party members at hand with an Elementalist just in case. The most common tactic is to 'Hit and Run' and 'Luring' to kill them all at once with no problem (especially if you are a Dealer). However, if you are cornered, killing mobs shall be easier said than done, so it is always adviced to keep your distance. Here,monsters can attack faster as stated, and their movement speed is boosted.