Short answer: Yes.
Longer answer: Just because something is released as open-source/under the GPL doesn’t mean you can’t put it behind a paywall. All WP themes have to be licensed GPL if you distribute them, because they are derived from the WordPress code, which is itself GPL, but you can still charge for it.
Your css and images, however, do not have to be released under the GPL – you could do a split license.
See this excellent article by Mark Jaquith: http://markjaquith.wordpress.com/2010/07/17/why-wordpress-themes-are-derivative-of-wordpress/