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)