Certainly; if you wrote the code and still hold copyright, you can license it under whatever terms you wish. You can GPL it, but then turn around and release a proprietary version. You have this right _if_ you're the copyright holder.

From what I can tell, the person who put this app on the App Store is not the copyright holder of GNU Go. (The FSF holds copyright to most of GNU Go's code, and they'd hardly offer an app on the App Store; and if they had, they could take it down without going through this mess.)

So what we have here is someone who took someone else's code (which was made available under GPL) and then released a DRMed version of it. The FSF is saying you don't get to do that.