So I've compiled a short list of the known recordings where Gertrude took live statement (which were not written and read out by her):
Episode 53- Walter Heller telling the story (in 1997) where he found an Archivist and an Archive in Alexanderia during WWII. A few months later there was an explosion in the vicinity, presumably by Gertrude's hands.
Episode 62- Mary Keay visiting Gertrude in July 3, 2008 to give her a 'gift' (Eric Delano's page) and a statement about the skin book and how she acquired it when she was young.
Episode 130- Lucia Wright in December 2008 telling her story of her encounter with a Flesh ritual run by Tom Haan (throwing truckloads of meat into a huge pit in Istanbul) that was stopped by an explosion caused once again by Gertrude.
Episode 145- Arthur Nolan pays Gertrude a visit in February 2009 and she out plays him and extracts a statement from him about raising Agnes Montague.
Episode 154- Eric Delano's ghost in July 21, 2008 telling her how to quit the institute and his story with Mary Keay.
It seems that for the most part, other than those who came to her to actively give a statement like Mary and Arthur, Gertrude tended not to take live statements.
Unless she had a very specific goal in mind of course.
With Eric, once she realized it was him, she mainly talked to him to get his secret on how to quit the archives. He asked her to take a statement as payment. With Lucia she wanted to make sure no survivor witnessed her sabotaging the Flesh ritual. And with Walter she wanted his help in locating an ancient archive and confirming her theory of the Archivist as a consistent Beholding role throughout history.
My theory is that she wanted to take a live statement from Lucia and Walter for an additional, very specific reason. She could have let them give her a written statement to get the information it seems she needed, since they have the same truth telling effect that talking does. With Lucia all she needed was for her to see Gertrude's face and recognize her and Walter was still unsure of the location even while talking to her so writing it down and then going over the maps with her as he did later would have yielded the same results.
She knew what live statements do. How they cause long term effects of nightmares both for them and for her. She didn't need to condemn them to such fate if that's all she took from them.
Unless getting those dreams was her main purpose. Imagine her appearing in Walter's dreams, walking with him through Alexanderia's alleys and knowing the location she needed to explode through first person sight. Imagine her standing next to Lucia as she carried meat from the trucks to the pits, knowing exactly who she saw and if Gertrude was actually seen by her.
Gertrude was calculative to the extreme and did everything for a reason. My theory is that she took these live statements because they gave her the dreams, because the dream were extremely useful in finding out that extra bit of reliable information she needed.
Just like with her assistants, she was willing to sacrifice her own humanity when she felt she had no other choice.