[Dutchman]

I'm learning grammar with Kim Tae's grammar guide, but in one of the excersices there was something i didn't understand about the ga particle.

アリス）　何　が　楽しい？
ボブ）　ゲーム　が　楽しい。

in the first sentence I understand why it needs to be ga, because you do not know the subject/object so you use ga but in the second one you know it's a game? or is it because you don't know wich game it is?