There is little reason to sell tether at a discount if you have significant amounts. You'd usually be better off just buying ether/bitcoin on Polo/BFX/etc and transferring to GDAX to sell directly for USD.

Same goes for buying it. In my experience, it only rises above $1.03 when liquidity is low and the BTC/USDT price diverges from BTC/USD enough to make arbitrage possible.