That is the problem, it entirely depends on how you use the GPL code and there is no definite legal precedence. If you use GPL code as part of your executable,

That is the problem, it entirely depends on how you use the GPL code and there is no definite legal precedence. If you use GPL code as part of your executable, the whole code used for building this executable falls under GPL. See for example the question, whether the non-GPL ZFS code can be linked against the GPL Linux kernel and the result distributed. See also all the GPL-only symbols the kernel exports. Another example is libreadline, which is put under GPL to force all applications using it to be licensed under GPL too. So there are clearly cases where using GPL code in your product can require the whole product be distributed under GPL. Of course, there are also cases, where this can be avoided. If I ship a product and I also ship emacs along with it, that doesn't influence my product. But if the product executable itself is derived from the emacs sources, it would.