This might be completely wrong but just off memory. When Gert was younger she saw Old Lace, and the parents injected her with something to make her forget and I think that’s what did it. I know that event happened but I’m not entirely sure if it’s a direct result of that. So don’t take my word for it

I believe the Yorkes used some type of injection to link Gert and Old Lace's brainwaves when she was a child.