Well it's very hard to explain. In case of your example sentence, it include some meaning of "because'. ex) I can't live without you, you are my heaven ; 난 너 없이 살수 없어. 넌 나의 천국이야. --> I can't live without you because you are my heaven. : 난 너없이 살 수 없어. 넌 나의 천국인걸... But normally it is shorten form for "~~ 것을" ex ) 새차 사는 걸(것을) 고려해 볼까? (Should we consider buying a new car?)