I'm so sorry you went through that and that the gmc responded the way they did.
What did you want from the gmc?
You should probably take legal advice from a specialist solicitor but I think (and I'm not legally trained) that there's probably a threshold in law for what counts as coercion and if they gave you time to consider then it may not be legally seen as coercion. Don't forget, coercive abuse only became recognised in law recently within a relationship.
Nowadays, I would expect such a situation to be referred to safeguading and a protocol followed, I have no idea if any such thing was involved when you went through this. If they followed all the steps as per their policies at the time, then I can't see they would be able to acknowledge wrong doing and I'd expect any change to this would need a strong legal challenge. If their policies have since changed for the better then any solicitor would ask what would be gained from attempting this.
It's deeply unfair and I understand that this must be incredibly difficult for you. It sounds as though as if it's had a huge impact on your life and I can't imagine what that has been like for you.
But I think, you need to be clear on what you want from the gmc. If you simply want acknowledgement so that you can obtain closure and move on, that's one thing. But any admission of liability opens them up to legal action and I'm not surprised they've taken the stance they have.
If what you're seeking is recognition by them that you were coerced and that's all then a solicitor could advise you what chances you had of seeking this through legal action while waiving any rights to redress. It seems like it would be a long shot though.
If you are only looking for closure then I hope you can find a way to get it without pursuing the gmc any further. So many people agree that it was wrong what you went through that even if your case is not strong enough to be tested in a court of law, I hope the agreement of others (including professionals) gives you the recognition you need.
The law isn't perfect and while the pursuit of justice is crucial to us all, when faced with overwhelming odds sometimes it's helpful to remember that just because the law is fallible, it doesn't lesson the injustice of what you went through or deny the coercion you faced.