Giglio answers a simple question: if the jury would trust a witness less knowing something, does the defense get to know it? The answer is yes.
Where it usually lives
Not in a file marked Giglio. It lives in internal email, draft reports, supervisor notes, and the contemporaneous assessments people write before a case becomes contested. That is why records productions matter so much: the impeachment material is almost never labeled.
The documented example
An investigating detective’s written assessment that the drug evidence would be hard to prove is potential impeachment of any later testimony asserting the opposite. See the 2022 email to felony review.