It's a fun discussion. *Drinks coffee*
GPL'ed licensed software means that the author cannot restrict the distribution or the modification of that software. That also means that you (the user) cannot restrict the software either if you distribute it.
If you use the software, modify it but do not distribute it then you do not have to share or make available your changes at all.
That's very free and we're all huge fans of GPL'ed software. Emil's theme's are very GPL'ed.
If you create a commercial theme off of Responsive like that you will absolutely need to credit the original author and you will not be able to restrict the distribution of that work in anyway shape or fashion.
Rather than starting with someone else's work you'll be better of creating your own theme from scratch. Trying to sell someone else's work is Just Not Cool at all.