This is the 193rd most frequent Korean word.
그가
"He" or "him" (subject form).
In this sentence, '그가' is a nominative pronoun meaning 'he', referring to the subject of the action.
그가 노래를 부르고 있다.
He is singing a song.
Here, '그가' serves as the subject of the relative clause, indicating the giver (he) of the book.
내가 그가 준 책을 읽었다.
I read the book that he gave me.
This usage positions '그가' as the subject of the embedded clause, which expresses a thought or opinion.