Do you ask for trading GRIN with fiat USD? If so, It's quite difficult for you to find a fiat market for alt coins always. Fiat - Crypto exchanges always support the buying and selling options for popular currencies like BTC and ETH for majority of the time. It would be a quite difficult hunt for you to find a way to sell or buy altcoins from a exchange using USD. Coinbase is one among the popular exchanges where you can buy and sell bitcoin with USD or other similar currencies but they support only a handful of popular altcoins. So does Kraken and Gemini exchanges as well.

Rather you can buy/sell GRIN using bitcoin in some centralized exchanges like Hotbit or Kucoin and then at a later point of time sell off your bitcoin through Coinbase, Kraken or Gemini.

If your query is to buy GRIN in a USA based crypto exchange you should trade GRIN in Bittrex exchange. The exchange has it's HQ in Seattle and is one among the most popular USA based exchanges which support GRIN.