Welcome to the forums, newcomer!
You're question would be better understood if you could parse a video or even a couple of images of why the GPU isn't fitting in the chassis. There's only so much that can be explained via text that you need the aid of other modes of communication.
The riser cable doesn't allow the GPU to be mounted outside the case, so the option of externally mounting it is out of the question. Also, that route defeats the purpose of buying a case in the first place. You could breadboard everything without using the H1.