Open source software means it is free as in freedom, not price. There is nothing in GPL that says you can't sell software.
I could take Q2A and sell it to you if I like. As long as I provide all the code upon request (which you would get anyway since it's a PHP app) then it is perfectly legal.
Similarly, if Gideon was selling Q2A through this site, but under GPL, then I could buy a copy and distribute it to anyone else I like, free of charge.
With dual licensing, you choose the option that matches what you want to do with it. In Q2A's case, since it is open source it uses the GPL license for CKeditor. If Q2A was being sold as closed-source software then Gideon would need to pay a fee to use one of their commercial licenses. Hope that explains a few things!