Short answer: Ideas, yes. Source code, no.
The source code is protected by copyright and the license only allows you to use the source code if you license the result as GPL. This doesn't block you from selling the result, but requires you to provide the source code to anyone you have sold the application to (and stops you from barring them passing the source or app on).
The ideas should be OK, as long as they're not otherwise protected (patents, essentially). There may be patents that allow an automatic license only for open source.
As always, please consult with a lawyer before taking legal advice from the internet. I am not one, so this is pure layman advice.