건 is contracted version of 것은 걸 is contracted version of 것을 게,거 is contracted version of 것이 건,걸,게,거 are mostly used in conversation as a a colloquial expression. 것은,것을,것이 are used with formal, honorific language or in document, papers.

tricky question even for korean haha Its not gonna be perfect but explain it at my best 것은 is simillar with greund when u wanna use verb as noun u can use this for example 먹는것은 즐겁다, 입는것은 어렵다 건 걸 것을 게 and actually rest of them seldom use it i think it also is used for making gerund form but the first one is most useful to remember because its the most frequently shown. I think u dont need to worry about grammar too much if u keep speaking korean with korean friend u get proper usage of those things I hope this answer would be helpful to u