The 5th Amendment protections against self-incrimination don't apply to Grand Jury testimony.
I don't believe that is true.
Sure they do. In fact, that's where these protections are usually applied (since prosecutors don't even call defendants to the witness stand by a prosecutor in criminal cases).
What you may be thinking here is that 5th Amendment protections don't apply in cases where the witness is given immunity from prosecution by the prosecutor.