In the earlier Genki chapters, Wa is used for questions.
However in Chapter 8, page 195, we are told to use Ga for questions using question words.
I am having difficulty understanding explicitly when to use Ga, since on page 60, we have
“Kore wa ikura desu ka?” (How much is this / This is how much?)
Would saying “Kore ga ikura desu ka” be any good?
So the rule is that you can’t put は on the question-word, not that it can’t appear anywhere in a sentence that’s a question.
This is because は is marking the topic, the “thing we know about that’s the background, not the focus of this statement”. By definition, question-words can’t be that.
I think the important thing here is that “kore” is not a “question word.” By which, I imagine they’re referring to things like “nani”, “doko”, etc. where you cannot use “wa” after them in a question.