그러니까 (geureonikka) means “so” or “therefore.” That is what the dictionary will tell you, and it is correct. In conversation, though, it does at least three other jobs that have little to do with cause and effect, and the most common of them is agreeing with someone.
In speech it often shrinks to 그니까 (geunikka) or even 근까 (geunkka). If you hear any of those, they are the same word.
The four jobs
1. So, therefore. The textbook meaning.
비가 와요. 그러니까 우산 가져가세요. (biga wayo. geureonikka usan gajyeogaseyo) = it’s raining, so take an umbrella
2. Exactly! I know, right? Agreement, usually emphatic.
A: 요즘 물가 진짜 너무 올랐어. B: 그니까! (A: yojeum mulga jinjja neomu ollasseo. B: geunikka!) = A: prices have gone up so much lately. B: I know!
This is one of the most frequent uses in casual speech. The other person said something you were already thinking, and 그니까 confirms it.
3. I mean, in other words. Rephrasing or clarifying.
그러니까 제 말은, 지금 당장은 어렵다는 거예요 (geureonikka je mareun, jigeum dangjangeun eoryeopdaneun geoyeyo) = what I mean is, it’s difficult right now
Used to restart a sentence that came out wrong, or to boil down what someone just said: 그러니까 결론은 안 된다는 거죠? (so in short, it’s a no?)
4. Filler. Like “so” or “like” at the start of an English sentence, with no logical content.
그러니까… 어제 그게… (geureonikka… eoje geuge…) = so… yesterday, that thing…
Buying time while you work out how to say it.
그러니까 vs 그래서
Both mean “so,” and learners often treat them as interchangeable. There is one hard difference.
그래서 (geuraeseo) cannot be followed by a command or a suggestion. 그러니까 can.
비가 와요. 그러니까 우산 가져가세요. (natural)
비가 와요. 그래서 우산 가져가세요. (unnatural)
This is the same rule as -아서 versus -(으)니까 on verbs, and 그래서 and 그러니까 are just those endings attached to 그렇다. If the next sentence tells someone to do something or proposes doing it together, use 그러니까.
그래서 is better for narrating what happened as a result. 비가 왔어요. 그래서 집에 있었어요. (it rained, so I stayed home.)
그러니까 vs 그러게
그러게 (geureoge) is the other common agreement word, and it overlaps with 그니까. The difference is subtle.
그니까! often sounds like strong agreement, as if you had been saying the same thing all along. 그러게 is softer and can carry a little resignation: “yeah, I know,” “tell me about it.”
Both can also go reproachful. 그러니까 내가 뭐랬어 and 그러게 내가 뭐랬어 both mean “see, what did I tell you?” Tone does most of the work there.
Where it does and does not go
그러니까 in its basic “so” meaning is fine everywhere, including formal speech and writing. It is a normal conjunction.
The agreement use, 그니까!, is casual. With someone you are on polite terms with, the natural version is 그러니까요 (geureonikkayo) or 맞아요 (majayo, that’s right).
The filler use is fine in speech in any register, but if you notice yourself starting every sentence with 그러니까, that is the Korean version of starting every sentence with “so,” and people will notice.
One thing I am unsure about
In writing, especially in comments and chats, I see 그니까, 그니깐, and 근까 used for the agreement sense, but I do not have a good sense of whether people perceive a difference between them. My guess is that they are just spelling variations of the same spoken word, chosen by habit.