This is the 3265th most frequent Korean word.
그다지
"Not so much; not particularly; not very."
In this sentence, '그다지' is used to describe the degree of the heat, indicating it is not excessively hot.
그다지 덥지 않아서 좋네요.
It's nice because it's not too hot.
Here, '그다지' expresses a moderate extent, implying that the taste was not exceedingly good.
그다지 맛있지는 않았지만 먹을 만했어요.
It wasn't particularly delicious, but it was edible.
'그다지' in this sentence conveys that the interest level of the book's content does not reach a high degree.