Not defending Garrison but hearsay is common in grand jury testimony. The rules of evidence do not apply to grand juries because they are not adversarial proceedings.
True, but if you read the GJ testimonies you'll see (in my opinion) that Garrison abused this rule. He didn't really question witnesses as much as try to guide or direct them. As in this bizarre exchange with Marina:
Q. Juror: "Marina, did the Secret Service say to you, Marina, it is better for you not to see Ruth Paine anymore because its not good for you, she might be saying things fo the CIA that might be detrimental to you, Marina?"
Marina: "No, because she....what is CIA?"
A. "Central Intelligence Agency."
A. "Could she be member?"
Garrison: "She probably is employee."
In fact, Marina thought the CIA was a reference to the American Civil Liberties Union, an organization that the Paines belonged too. She testfiied that the first time she had ever heard of the CIA was AFTER the assassination.
Her strange (very) GJ testimony is here:
https://www.maryferrell.org/showDoc.html?docId=1203&search=CIA#relPageId=72&tab=page