Though I'm in no place to diagnose anything of any sort, you may find it helpful to look up "health anxiety" because from the posts I've read of yours, it sounds like you may have at least some traits of that, and often it can have elements of GAD.
I have the exact same fear as you - I worry that I will black out and die. There doesnt even have to be anything making me think that, I just think it could happen randomly. But what you need to know about this is that with blacking out, a common cause is lack of oxygen, and though you may not feel like you're getting enough oxygen, you most probably are. In fact, you're probably getting too much, and this inbalance of oxygen and carbon dioxide is what causes the feelings of dreamworlds and unreality. Because of that, it's an extremely common symptom of anxiety. Also, with blacking out, when people black out, more often than not certainly, they do not die. Blacking out is the body's way of getting the body lying flat so that blood, oxygen, etc. can reach all important parts of the body, therefore protecting it, making you LESS likely to die.
It's a really scary place to be, but if you suffer from anxiety, then all manner of symptoms can be created by anxiety, so it's much, much, much more likely that it's just anxiety, not anything sinister.
