그걸 보고

seeing that

→ 그것+을
phrase Rare #26,625
seeing that 그걸 means “that” or “that thing,” and 보고 is the connective form of 보다, meaning “seeing” or “after seeing.” It can introduce an action that follows seeing something.
KO: 그걸 보고 정말 놀랐어요.
EN: I was really surprised when I saw that.

한번은 테이블 위에 누워서 아빠가 그걸 보고 말했잖아

One time, I was lying on the table, and my dad saw me and said...

— 더 위치 (2016)