Bittrex does not currently give GAS rewards. However, Binance and Kucoin do give you GAS while you're still holding NEO on the exchange. There is speculation that Poloniex may eventually also give GAS rewards, since they already list GAS but not NEO, but for now that is only speculation.
In general I wouldn't recommend holding NEO on any exchange unless you either have very little or are day trading, but if you really want to then Binance and Kucoin are currently your only options.