g is "by default" unvoiced, but it becomes voiced between other voiced sounds (vowels or nasals). So in gagu "가구" (furniture), the first g is unvoiced but the second is voiced.

What complicates matter is that, in English, word-first g (as in "game") can become partially unvoiced, so g in English "game" is actually fairly close to the first g of "gagu"!