Desposit USD in Coinbase -> Buy Bitcoin on GDAX -> Send to Bittrex -> Buy Alt Coin on Bittrex
Sell Alt Coin for Bitcoin on Bittrex -> Send bitcoins to Coinbase (GDAX) -> Sell bitcoin for USD -> Withrdaw from Coinbase
Is this the correct flow? And if so, since I'm making 2 conversions (USD <-> Bitcoin <-> Altcoin), couldn't there be a major fluctuation before I complete my trade? For example, the Altcoin goes way up, I convert it to Bitcoin, then Bitcoin happens to go way down, then I convert it to USD, ultimately losing money?
You are exposed to risk for the longest period of time when you send bitcoins between coinbase and bittrex. The other steps are quick. In general, the BTC/USD rate doesn't change quickly enough for that to be a major concern.