걸 다

all of it

→ 것+을
phrase Uncommon #10,103
all of it 걸 is a contraction of 것을, meaning “thing” as an object; 다 means “all.” Together, they emphasize that the whole thing is included.
KO: 그 얘기를 걸 다 기억해요.
EN: I remember all of it.

내가 알고 있는 걸 다 얘기할 거 예요

I’m going to tell you everything I know.

— 이중 배상 (1944)

야, 우리 예승이 때문에 형님 책 읽는 걸 다 보게 생겼다, 야

Hey, thanks to our Yeseung, I might actually get to see you reading a book, bro.

— Miracle in Cell No. 7 (2013)