If you receive an open source application from your employer, it hasn't been distributed to you, but to your employer, so you haven't received any rights through that distribution. (However, I have been told that handing software to contractors might be different, so the employer should be careful).
Even if you had rights, you don't have the source code. Even if you are physically capable of doing so, you are not allowed to just take someone else's source code. You might have the right to ask for it, but until you get it, you don't have it. And if you don't have the source code, you are incapable of legally distributing the software under the GPL license because you cannot possible fulfil your obligations.
Only if you are in a position in the company where you could legally take the company's source code and hand it to someone, only then can you legally publish anything under the GPL license. Even then you could be fired if you are acting against your obligations as a paid employee.
“Exactly which law” - first, copyright law. Second, anti-hacking laws. Third, employment contracts. Fourth, you don’t have any rights to the code, so you can’t legally give it away under GPL license. In summary you will get fired, they will get you for unauthorised access to a protected computer, and everyone will go after you for copyright infringement.