Bittrex seems to be the only major exchange allowing US traders to use their GRIN pairs, though if you haven't used the exchange before and you're interested I suggest looking at this page to see if you're eligible to sign up based on what state you live in. As the member above said, another easy way to move your assets into fiat or BTC is to trade on another exchange that doesn't require KYC and then transfer your funds to more US-compatible exchange that accepts the coin and also allows for fiat withdrawals.

EDIT--

KuCoin doesn't explicitly state that they don't allow US residents to trade on their platform (though they don't do KYC with US residents, thus you don't have access to the full platform), but like with most other similar platforms over the years, that can change at any time. Without submitting KYC, you can withdraw 2 BTC daily like with Binance.