Am I obligated to return the money my aunt gave me to buy an apartment, knowing that I accepted it as a gift and she did not consider it a debt at the time, and six years have passed since then?

You are not required to repay the amount to your aunt unless there is evidence, either explicit or circumstantial, that it was a loan. This is because, when the giver and receiver disagree, and the giver claims it was a loan while the receiver states it was a gift, the receiver's word is accepted, as the apparent situation supports their claim.