What do these mean?
What's the meaning of ''걸' and when\how do you use it?
and if you have "'는 걸" at the end of a sentence, is it the same as "걸"?

Also, I know "는구나" is for exclamations but.. I don't entirely get how you use it? can anyone explain it better or give me sentences

Question

What do these mean? 
What's the meaning of ''걸'  and when\how do you use it?
and if you have "'는 걸" at the end of a sentence, is it the same as "걸"?

Also, I know "는구나" is for exclamations but.. I don't entirely get how you use it? can anyone explain it better or give me sentences >

Answer

when you watch something and understand what that is, we use ~는구나 or 걸.

for example,
when you see owesome sports star like 김연아.
멋있다 = > 멋있는 걸!
or 멋있다 => 멋있구나 (는 should be out in the sentense  because of grammatical reason)

Answer

걸 is so widely used in spoken Korean.

#1
A : 야! 일어나서 밥 먹어!
B : 아씨... 나 어제 늦게 들어왔잖아. 잘 자고 있는 걸 왜 깨우고 그래-
잘 자고 있는 걸 = who was sleeping well = B

#2
A : 나 짜증나게 해놓고 지금 밥이 넘어가냐?
B : 아, 배고픈 걸 어쩌라고?
배고픈 걸 a hungry state

I use -ㄴ/는데, -다 for exclamations. 
-오, 너 오늘 좀 멋있는데?
-이야, 죽인다!(slang; *ucking great/cool/nice)

-ㄴ/는 걸(at the end of the sentence) sounds old-fashioned to me.

About -(는)구나... in my personal lanugage, it has two usages. (I don't use it for exclamations)

#1 (cellphone conversation)
A : 야, 거기 ㅇㅇ(A and B's friend and hiding at A's home) 있지? 
B : 어어~어? 아닌데? (speaking in a hoarse tone)
A : 아니긴 뭐가 아니야? 거기 있구나? S/he is there, isn't s/he?

#2 
A : (singing terribly)
B : 아주 작곡을 하는구나. You're really re-writing the music(sarcastic; B is saying A's song is out of tune)