This question is specifically addressed in the GPL FAQ, and it says you're allowed to use GPLed software inside a company without legally distributing it. You have no obligation to release either source or binary outside the company.
You're talking about what Richard Stallman (the person behind the Gnu movement) calls "private software". For private software, any license that allows you to use the code works, because you're not distributing it. Both the Free Software Foundation and the Open Source Initiative maintain that it should always be possible to use software privately.
