Maggie,
You are done because you say you, and this isn't a gender issue.
While this may be gender related because of your position, it could easily apply to anything in your life. What if you hadn't transitioned, there is a chance you would question seeing her would you not? I think the therapist was too quick to put blame on GID. It's not your GID being the problem here, it's your relationship with her which has been strained for many years.
While I know therapists in general are there to help, some are also looking to line their pockets. I also wonder if maybe it was a bit of a power play for his benefit. You being "done" and an author, he may see you as a threat to him leading the group, he had to re-establish dominance and show the others that they may need more therapy than they think. There is a lot that could be read into how he reacted, all of them seem wrong.
I can't tell you if you should or should not see her. Ask yourself what you hope to accomplish, what will likely occur, and then make a decision. What you are facing is a relationship decision, not a gender issue.