So far example, dreadful story I am aware of is a man who raped a number of young men whom he drugged. Many of whom were not aware they had been raped. His intention was certainly not to cause them physical harm or even psychological harm. Although of course, this did happen when they became aware of the rapes (after footage he kept was discovered by the police) - it was clearly not his intention that they should ever become aware of the rapes.
Now you're quite right - in the above instance these rapes were about power. Clearly the man in the above example had a sexual interest in raping somebody drugged and unconscious - and that was probably rooted in the extreme power dynamic of that situation. Nevertheless it was that sexual interest which motivated him not an intention to cause harm to the victims.
Another hypothetical example might be a healthcare worker has sex with a patient in a permanently unconscious, vegetative state - if they use lubricant, a condom there may be no physical harm. If the person is incapable of becoming aware they were raped there is no psychological harm. Is this GBH? What it definitely is, is rape, an obscene crime which should be punished.
I think it's a misconception that rape isn't about sex, it very often is about sex.
In this instance the victim was not known to her attacker and was unconscious. He wasn't doing this to punish her, enjoy her suffering or because of some relationship they had (I assume on the information available). He did it because the situation of her being vulnerable and unconscious was sexually exciting to him. I know that's hideous but there It is. It was about sex, and it was also about power.
I don't say any of this to minimise or exonerate, these are heinous crimes. Nevertheless I don't think it is helpful to categorise rape as GBH. It loses focus on the essence of rape which is penetration without consent, whether harm can be demonstrated or not.