GReddy is a sub-brand of Trust Company Ltd., a Japanese automotive aftermarket company that specialises in performance tuning parts for cars.
The brand is featured in Gran Turismo Sport and Gran Turismo 7 (albeit appearing as a standalone manufacturer only on the latter game) as the manufacturer of a unique Datsun Z which won the 2015 SEMA Gran Turismo Awards top prize.
In GT7, GReddy is listed as an American manufacturer, though for event restriction purposes, the only car under the brand is instead eligible to compete in events restricting participation to Japanese cars instead of American ones. This might be because their only car is a derivative of a car originally produced by a Japanese manufacturer.