Alright, I'll explain the 4 last.

Silent Club - Silent Club was a level created in 1.6 which was the silent version of the main level Clubstep.

pornographic levels - Due to the addition of img to GD hacks, there would be a lot of pornographic and explicit levels.

Element 111 Rg - On GW, a korean cafe about GD on Naver, GW Arco would create a series of Element levels based on their element name and number on the periodic table. There would have been a total of 120 levels but of course all of them would be finished or some would be deleted, and each 5 to 10 elements would be assigned to respective creators that would volunteer. The elements from 115 to 120 would be assigned to DarkX and he would only create Element 111 Rg. Althought other element levels would be simple for the community and was just maybe based around the element itself, Element 111 Rg would be one of the hardest levels