I have never personally traded with G6 but I have seen their contracts and learned of their business structure and they are set up as they should be. Don't go into any firm expecting them to tell you what to buy/sell and you can just ride someone's coat tails and not have to put in the work. Trading is hard and will only work after years and years of blood, sweat and tears. Anyone telling you something else is selling something.
I have never heard anything negative about G6 and they are a registered firm.