Although the GPL allows for commercial distribution, provided that the source code and copyright notice is retained, there are a number of contributions included with Danger from the Deep that are not licensed under the GPL.
These include, but are not limited to; the 2D and 3D artwork, the music/soundtrack and other miscellaneous audio/video/multimedia sequences. These are licensed under the CC At-Nc-Nd 2.0/2.5 and cannot be sold without prior consent from the copyright holders.