The issue with regard to what must be made GPL and what must not be is unclear and will likely remain that way for some time.
The two points of view are:
1. WP Themes derive from WP and so must be GPL.
2. Themes are original creations and so are not derivative except insofar as they use certain WP-specific functions and such to work.
Both points of view have some backing behind them.
What's not stated is that arguing them is more than a little pointless, because here's the real truth of the matter: Unless the copyright holder(s) of WordPress wish to assert their copyright and take somebody to court over it, it's a bit of a moot issue. Nobody else can call them out on it, since *if* a theme author is infringing copyright by not releasing their theme under GPL, then they're infringing the copyright of WordPress itself.
So as long as Matt and the other WordPress copyright holders don't call lawyers, there's really nothing to discuss.
Back on the original topic:
If the theme is GPL, then of course, you're free to rerelease.
If not, or if it's unclear, then you should try to get somebody's permission first.
If the theme, as it is in this case, is derived from another theme in the first place, then look for a license there. If that one is GPL, then so is this new one, and you're good to go.
Also, consider the possible consequences. Worst case: Somebody says that you're infringing their copyright, you remove the theme, done. It's highly unlikely that any court action will happen, and if it did, it's highly unlikely that they'd be able to prove any damages, since the original theme was a) free and b) non-functional before you fixed it up and rereleased it.