Unfortunately I wasn’t able to hear anything during that panel. The sound was horrible. But if she indeed said that, I don’t think it has anything to do with David. They’ve both admitted they’ve been quite toxic to each other for a period of time, even if “toxic” might be a little too much to describe what happened between them, but they’ve also both admitted how much they love each other, that they have a unique and great relationship today, that they’re soulmate and will be linked to each other forever. You don’t call toxic someone you said you’ll be buried right next to him when you die, even if it was a joke.
Unfortunately, it seems like Gillian have had a few toxic relationships in her life, so either she meant a friend, a member of her family or even a lover, I don’t think she had David in mind for once second.