When you say GPL, do you mean copy left? You could probably use CC-BY-SA for that, but I'd recommend reading this article by John Wilbanks at Science Commons

When you say GPL, do you mean copy left? You could probably use CC-BY-SA for that, but I'd recommend reading this article by John Wilbanks at Science Commons http://scienceblogs.com/commonknowledge/2010/06/open_data_and_creative_commons.php http://scienceblogs.com/commonknowledge/2010/06/open_data_an... as well as the Panton Principles http://pantonprinciples.org/ http://pantonprinciples.org/ In general, like with certain software tools and libraries a copy left license might not be the best choice, which is part of where John comes from.