If the code was originally released under GPL, then any modified version of the code, if it is released at all, has to be released under GPL as well. The concept is referred to as 'copyleft' and is set up to protect authors of open source software from having their work exploited commercially by third parties. There's a lot of info available under http://www.gnu.org/licenses/gpl-faq.html.

In this case, we want to include GPL-licensed code in another GPL-licensed project. That's exactly the sort of thing that is allowed and even encouraged, but we'd generally have to keep an attribution around. Maybe we could mention explicitly that the code has been adapted from Chroma?