There’s no way in the world this person could be David. I can accept the idea that maybe they’ve been toxic for each other for a few years, in the end of the 90s, even if I think the word “toxic” is too much for what happened between them. But several decades? Clearly not. They’ve actually spent the last decade making it up for each other, saying the nicest and sweetest things about each other and about their relationship. Just look at all the sweet quotes we have had in the last 10 years.
Besides that, purely PR wise, I don’t think she would talk about it if it was someone we know, and even less if it was David. She said she had to think a lot about what to reveal about herself and what to keep private. So if this person was someone who’s under the public eye, I think she would have kept it to herself. It’s just pure logic: if it was David and if she choses to talk about him in those words, it means they’ve fought and they’re done with each other. In this case, what would stop him for standing up for himself and tell the same things about her and jeopardize her book and her reputation? That’s a risk every publicist and Gillian herself would have taken into consideration if David or any well known person was the toxic one.

We actually know very little about Gillian’s private life, her friends or her family, so in my opinion, it’s probably no one we know.