I just sold my Bitcoin Gold coins a few hours ago. I actually wanted to sell them since I got them (I mean I didn't mean or have the intentions to hold on that shitcoin).
The easiest way so far was using Coinomi. Download the app from Google play store. And then choose bitcoin Gold. You'll then have an option to sweep an old wallet by inserting your private keys to any addresses from which you want to import your coins.
I did that and then sent all the coins to Bittrex's wallet obviously to sell them for btc.
From your new BTG Coinomi wallet's Overview page, click on .../Sweep Wallet, and enter the private key