It's hard to decide if the visceral reaction to mentioning him is due to the knowledge of the many terrible things he has done or had done at his command, or if it's a spillover from the memory of D's feelings.

As long as he isn't here, Gaia's memories won't be a danger that will draw his attention. "I won't say anything unless it's an emergency." D doesn't often volunteer information anyway, but he won't give his word he'll say nothing when there's a chance it may become necessary. There are easy enough ways around that even if he did say it, but he'd rather not complicate situations unnecessarily.

If this gets worse or doesn't approve, he may have to help more directly. They are his memories after all, they'll be easy for his Left Hand to locate even in someone else's mind.