Ok, i know people will disagree with me, but if we are talking about gothic metal (not just gothic music in general) and doom metal, then I always feel that doom has to be slow, where-as gothic metal can be slow, but can also be faster. Fast songs just dont sound 'doom' like to me. i know there are other differences too, but this is one of the main differences, i feel. As i've stated this is my opinion, and i know people will disagree.