Re: *Whose* code?
One of the conditions of the GPL, as you doubtless well know, is that you are enjoined to publish your own modifications to that code freely for the benefit of others.
Fallacy. You are only required to share your modifications if you distribute the modified version. If the "you" is a commercial entity, and you only use the modified software in-house, then there is no requirement to share your modifications with anyone.