The general idea is to have a site that can track various projects that release their own copy of GPL (especially, but other licenses as well) software. So Apple ships their version of {bash, etc.}, and so does your router, and someone else's refrigerator. And this site ("GPL Hub?") just links to all the places that are hosting their own version of {bash, etc.}. Maybe it could even show how they've diverged, although that might not be so easy.
Recently it has become very trendy to host all code in one centralized location like GitHub. But GitHub isn't really that great at showing you the "forks graph" anyway. Many companies that ship GPL code have archives like this available on their own websites. This could help increase visibility and give users an easier time finding the source code. I'd love to know how many copies of different GPL projects are out in the wild. Maybe for the companies that don't publicly host their GPL code, someone who makes a successful GPL request could share the code on this same site.
Those are some of the ideas, I'd still love to finish the post sometime but it's a bit too vague for me to say anything concrete yet. Any direction to related works or explanations of why this is a bad (or good) idea would be much appreciated!