Hi anon! Yes I do believe they were together in the 90s, probably not for a long period of time, but I think they dated in addition to be fooling around for years.
In my mind and in their own words, they are soulmates and they belong together but they’ve never been in sync on this. I think in the 90s, Gillian was ready to embrace the fact that she loved him and that they should be together but David was not, he wanted to date Ivy League successful actresses. In other words, he felt like he deserved better, so he pushed his feelings for her away and hurt her.
I think they kept fooling around from time to time, seeing each other “on rare and pleasant occasions” (wink wink)
In 2011-12, he finally came at peace with his feelings and decided it was time to give it a real go, she was single, so was he, so they tried. But somehow, Gillian started to be like “maybe I deserve better too, after all!” And even if she had feelings for him, she always did, she wanted to be with successful pompous Brits like Mark or Peter Morgan. So she pushed her feelings away and hurt him back.
And here we are, with two broken lovebirds who belong together, who know they belong together, and yet refuse to be together. For now.
Disclaimer: this is just my take on things, it doesn’t have to make sense for anyone else but me!