그냥 and ㄹ(을) 뿐이다 has common part in terms of meaning: just But if you study them deeply, 그냥 has the meaning of “without any reason/cause/context, (generally) just ”, while ㄹ(을) 뿐이다 means <sth is the only thing> So it makes slight nuance difference: 어제 뭐했어? What did you do yesterday? 그냥 잤어. I just slept. (Without any context) 왜 모든 여자들이 널 좋아해? Why do all the women like you? 몰라. 나는 웃었을 뿐이야. I don’t know. The only thing I did was smiling. 나한테는 너뿐이야. To me, you’re the only one (I love) 그냥 너를 좋아해. I just love you (without any cause/ reason) Hope this works for you.