KG has a major role in the flick, which is about a degenerate sports gambler who gets into major problems when his debts spiral out of control.
Garnett plays himself ... and we had heard that during down time on the set, he stayed in his role and just murdered people on a basketball court they used in the movie.
So, when we saw "Uncut Gems" director Josh Safdie at the Allice Tulley Center in L.A. on Thursday, we asked him about the rumors ... and yeah, it's all true baby!