그제서야 (geu-je-seo-ya)/그제야 (geu-je-ya)/그때서야 (geu-ddae-seo-ya) – at last/finally (at that/this time)
All of the above have more or less the same meaning and you would typically insert them in the middle of a sentence.
You can use this phrase to describe how someone took a long time to realise or know of something that has been a fact for a long time.
우리 얘기한 지 2 시간이 되어서 너 그제서야 입을 열고 말해?
We have been talking for two hours and you finally (at this time) opened your mouth to talk?
사람들은 건강을 잃고 나서 그제야 그 고마움을 안다.
When people lose their health, they would finally (at that time) know to be thankful of good health.
or Naver Dictionary’s translation: People do not know the blessing of health until they lose it.