na-adjective, interjection
Casual interjection or adjectival noun used to express approval, satisfaction, or that something is good. Often used in informal conversation, similar to English 'good' or 'nice'.
from English good
このアイデア、グーだね。
This idea is good, isn't it?
テストの結果はグーだった。
The test result was good.
グー！
Good! Keep it up.