I have to concur that the dreams are an expression of her fears about you tbh. I know you probably realise this and don't need it spelling out but she's already been abandoned in her head by her dad. she also faces the very real fear that her mum will also leave her forever (I know this is hard to stomach) I felt like this, my mum wasn't ill (I was also older, so able to rationalize more).
In her mind, the gf has taken her dad away, and now is ALSO taking her mum away. I realise this makes no se se, but 8yos don't always make sense. Your GP is being unhelpful imo, your Dd needs to speak to a professional.
Contact needs to continue so that your DD can see that she isn't being abandoned. Maybe your ex could have her more often. You can't dictate about the gf, because simply, that is a convenient outlet for your DDs very real fears, but its not the real focus.
Do you mind me asking what your prognosis is? If its good, I suggest you take her to a few appointments with you so that she can ask the doctors to explain what is happening to her. The cancer, like the gf, is unseen, and probably only talked about away from her, or in private conversations she isn't supposed to hear (like the gf). Possibly in effect the gf has become the cancer in DDs head.
Fact is your is ex going to move on and have gfs, that can't be stopped, nor should it, stopping contact will only reinforce the fact she only has one parent left. I think you need to discuss the possibility of increased contact and his having a good amount of contact (if not residency) should the worst happen to you.