So someone you work with broke (or attempted to break? It's unclear whether the product you're referring to was released) the law by using GPL code in a closed source commercial product? This is done right improper (and illegal) for the developer who used the GPL software. Not the creator who choose the GPL likely to specifically prevent what you described.
People tend to understand open source software as software that is free to use however you want. This is not what GPL software is though. It's entended to be an ecosystem of software that is open and free to use as long as you are also willing to share the software you create using the ecosystem.
What is a situation where liscensing your software as GPL is "questionable if not down right improper"?
The entire point of the GPL is to spread the adoption of free as in freedom software. If you aren't willing to share your project with the community you are not allowed to use the community's work.