Re: "Read more and try to understand when and where the GPL is an issue"
> I know exactly what the GPL is,
No you don't.
There is no such thing as "exactitude" in law. This is something that has been emphasised by each and every one of my lecturers (I took a few law subjects in addition to my core studies).
> and how much it make developing closed source commercial software on Linux difficult
Nonsense. Every major Linux vendor, starting with Google, have a significant portfolio of closed-source products.