Why would you GPL the code in the first place if you didn't want other people using it? It's perfectly within the license for someone to do basically whatever they want with GPL code as long as they're not redistributing it. That includes using it for the internal operations of a Fortune 500 company, using it to run a dictatorship, or building a SaaS business on top of it. If you don't want people to "steal" your code, the GPL isn't the right license.