As we all know, the GNU General Public License (GPL) is a "copyleft" source license: which means it is designed not merely to make the code to which it is initially applied free and preserve its freedom, but moreover to "catalyze" the creation of new free code by requiring that any original works that use any copyrightable portion - no matter how small - of the GPL code in them, to also be released as free/open code under the GPL, in their entirety, and not merely the parts specifically depedent upon it.
It seems this only applies to the case where you directly mix code, i.e. where you actually take a piece of code from the GPL program and put it physically into the source code files of your original program, and perhaps also to linking to it as a library (which is why the LGPL exists, to exempt this particular use case).
What happens if you try to make an even "looser" coupling? In particular, if you were to compile all the GPL code and any modifications thereto as a totally separate executable program that can run on its own, independently of the original program, but yet which the original program communicates with as a spawn process, while still not containing any actual GPL code, whether directly in its source code or indirectly included by linking, merely the process calls alone, and as a separate program could in theory be used just same by any other program without modification. And moreover, when you distribute the proprietary package, this stand-alone program comes with its complete source code available under GPL terms. Would this be legal or not? If not, what exactly in the GPL and/or copyright laws forbid it?