there IS this price differnce because people can NOT do arbitrage without problem. This is simpoly because it is USDT and USD.
You can not sent USDT to gdax/coinbase, so you need to convert the USDT to USD (or other way round) and send them via wire, which is expensive and it is not easy to exchange USD/USDT.
Do you now where to buy USDT with USD? I don't have any crypto just USD.
You can probably buy it at tether.io, although I never tried it. Bitfinex used to offer both USD and USDT, but I think you can only use USD now. I wouldn't recommend you buying any though, because there is still a lot of conspiracy around them, and until we know what's true and what's fake I would stay away from them. Everyone is asking for an audit for a long time, and until I see one where they can prove that they really have all those USD, to back their USDT, I will not use it.