How to properly license a WordPress theme?

I’ve built a WordPress theme on an existing theme designed specifically for creating themes (like a framework but not as extensive). And after several hours of coding and testing I’m kind of wanting to sell it. However it seems to me that the original theme is GPL licensed. So I’m wondering if it is legal for me to sell it and keep it under the GPL license or not? (If not I’ll just give it away as a freebie to promote my website.)