GDAX, while it is a cryptocurrency exchange (run by coinbase), currently does not trade Safex to my knowledge.
If I were you, I would trade my BTC in GDAX to LTC or Ethereum and then transfer it to Bittrex. The reason for that is BTC network fees are currently very high, so you will save money if you do it that way.
Make sure if you trade your BTC at GDAX for LTC or Ethereum, that you transfer the LTC to your Bittrex LTC wallet and/or your Ethereum into your Bittrex Ethereum wallet. If you try to send LTC to your BTC wallet address on Bittrex it will be gone forever. Hopefully that makes sense.