General > Reset > Reset All Settings
Thats the only way I’ve heard of fixing it, hopefully Apple comes up with a more elegant way, because it’s downright annoying and absolutely NOT user friendly.
In iOS 6, if you logged out and logged back into GC, it would be re-enabled. But that functionality was removed in iOS 7 for some reason.
Edit:
I read in another SO question, that this works properly (log-out, log-in re-enables GC) outside of a sandbox environment.