That said, try not to be such a twit about saying GPL stuff is unusable for commercial projects. If you want to write commercial closed-source programs, there is absolutely nothing that prevents you from using GPL tools to do so. gcc, gdb, and the rest of the toolchain you need for creating software are all open source, and you can use them as much as you like to create things that aren't.
It goes further than that. Reference: http://blog.milkingthegnu.org/2008/04/gpl-for-dummies.html and http://www.gnu.org/licenses/gpl-faq.html#GPLIncompatibleLibs. One usage scenario would be to write your game under GPL as a fully playable but limited trial version, the proprietary plugin expands that into the full game. This is perfectly legal using a GPLv3 section 7 exception clause. As I mentioned in an earlier post, also possible is GPL client with proprietary server, where server can be an actual game server, a simple content server, or something in between. There are numerous opportunities along these lines if you bother to look for them.
I've come to the conclusion that the thing most dangerous about the GPL, is the people who post advocating it.
Your usage scenario makes no sense. Let me explain. If you write a game and release it under the GPL, you already own the full copyright and have the right to use the source licensed otherwise. That includes as a closed amended source version, or even with closed source plugin game expansion. There's nothing legally stopping you from doing that. The GPL and releasing the source is irrelevant.
Then there's section 7 of the GPLv3. The only way it would apply, was if you could add permissions allowing you to take changes people make to your GPL released code (which in turn are GPL licensed if they release them) back into your non-GPL version. In which case their code doesn't get the benefits of the GPL, because you wish to do to them, what you don't wish them to do to you.
And Bear, don't be unpleasant bullying people into accepting the GPL with ridiculous arguments. You can make a hat out of strawberries, but it's a dumb idea. And so, just because you can contrive to release a commercial game with GPLed code doesn't make it a good idea. Few people do it for a reason. Using common tools like gcc and gdb is one thing, as they are so well developed that it is unlikely you will be patching them and releasing changes. But using someone's niche custom library, means you are more than likely to bug fix it and modify it to remove eccentricities. And of course, link it in. It becomes a completely different kettle of fish. A relevant one, compared to the irrelevant gdb/gcc one.
The fact is that this topic is strewn with vague claims. The GPL is inclusive of other licensing - well, only in a viral sense that it forces the code in those other licenses to be open sourced as well. Which is why it is called a viral license. And yet, we have another pro-GPL supporter in the topic claiming it's not viral.
There's also an irrational fear by some pro-GPL posters. That some mystical evil-doers will swoop in and take their non-GPL code and make something out of it they don't want made. People are already doing this. My satellite receiver firmware has GPL code linked into it, and it has been reported to the GPL violations mailing list years and years ago. But nothing ever happened. The GPL offers no real protection above what a more free and permissive license like the MIT license offers. Not unless the FSF can earn some publicity.
Again, I use the GPL for software I have released to others, and use software licensed under the GPL released by others. The GPL is a valid choice if you choose to use it with all the facts in mind. But the problem is that lots of people want other people to use it, and they either don't understand it fully, are rude in their pushing of it (for example calling people twits) or make disingenous claims so as to foist it on others.
Please stop bullying people into using the GPL with false claims, tangentially related claims and name calling. I am sure we all understand how it's viral nature is good, in the way it is intended. But it isn't an all purpose solution which can be as easily manipulated to suit, as many pro-GPL posters here claim.
It's no wonder people are afraid to even read GPL code, with so much disinformation and confusion.