Hi OP, I work in a similar setting to yours and I think HR will take it seriously enough to want to investigate, mostly because the gap in seniority between the two of you indicates that there was a power imbalance and the relationship was not equal.
Do be honest with them and tell them that you got in the relationship willingly but that things got difficult and he started harassing you after you tried to end it. HR will not care whether or not he is married, but they will definitely care about the fact that he was 4x your senior as you say.
They will probably want to get as much detail as possible from you and will then decide whether they will interview him or others (like your team leader). This may be a formal investigation or a “fact finder” which is the stage just before (I don’t actually work in HR and the terms may vary between organisations but this is from my experience in my Trust).
The GMC will only get involved if the matter is reported to the police (and police have enough evidence to charge), if your employer decides that the matter is too serious for internal investigation and decide to refer to the GMC (unlikely to be the case based on what you said, unless they have similar complaints about him from other people), or if you decide to complain to the GMC yourself.
I don’t want to be patronising but when I was in my 20s I found it difficult to deal with situations like this too. The compliments and attention can feel very validating, and for someone with a people-pleasing personality like you say you are, it’s easy to fall into situations that you didn’t really want to, because you feel guilty about saying no and upsetting someone. Just be aware that his suffering isn’t about you, he’s probably having a midlife crisis and wanted an excuse to get out of one relationship to the comfort of another one. He will 100% recover from this. And even if he decides to leave his wife (which may be a good thing for her, all things considered), that does NOT mean that he will be “your responsibility”! You should be firm and not let yourself be coerced into a relationship out of guilt. He is probably on a good salary (I’m assuming he is a consultant or clinical director) and will definitely manage to get his own place if needed.
Please cut off all contact now and be open with HR about what happened and about your concerns (his turning up to your place unannounced late at night and trying to coerce you into continuing a relationship should be enough to worry them). It’s unlikely that he will be sacked over this (if that’s your worry) unless he shows total disregard for the investigation and total lack of insight. Doctors and other senior healthcare workers have got away with a slap on the wrist for worse things, so don’t let your feelings of guilt and fear for his career stop you from looking after yourself.