No. Not only is credibility never tested in a GJ because the witnesses are not cross examined. Bu Prosecutors will put forth evidence that they know would NEVER make it before a jury.
Under Oregon Law, a Grand Jury's job is to determine whether
the evidence presented to it, if uncontradicted or unexplained at trial, is sufficient to support a conviction. ORS 132.390. Generally speaking, the rules of evidence apply, ORS 132.320, and national standards remind prosecutors to respect the panel’s independence and to not unduly influence its decisions.
http://www.osbar.org/publications/bulletin/04jul/secrecy.html
IMO, the LS is a great example of how the GJ works. Alledgely the LS is being called as a witness to Terri's character. The LS is not being called
to testify about character traits that make Terri who she is. He is being called specifically to talk about his allegation that Terri tried to hire him to kill Kaine. They want the GJ to come to the conclusion that if Terri's is willing to hire someone to kill her husband, she is willing to harm to Kyron.
IMO, in a case about Kyron, the LS would NEVER be allowed to testify at trial. You cannot used an uncharged different crime to convict someone on another crime.