Why isn't it 가구가 없습니다?
I just replied to your comment but I forgot to press the "REPLY" button below your comment
Maybe in a context where you're asking about an apartment's condition, whether it's furnished or not, and this could be the answer to that.
But yeah, without context it is really hard to say why they phrased it this way.
가구는 없다. = There is no furniture
아무 가구도 없다. There is not any furniture
=> There is no furniture
가구기 없습니다 가 맞음...