All assuming the company has modified the GPL licensed software, so they have a copyright in the code: The company hasn't published the software, therefore it has no obligations that would arise from publishing it.
The thief committed copyright infringement by copying without permission of the copyright holder. He or she may have committed all kinds of other crimes, like hacking into the company's computers, violation of trade secrets and so on, depending on the situation.
The thief has no permission to make further copies, and no permission to license it under the GPL. That means he or she is creating a legal mess. Anyone downloading the software believes it is GPL licensed, but it isn't. A downloader has no right to use it. This would be different if the complete source code had been GPL licensed (if the company had used an unmodified copy of GPL licensed code).