Use Creative Commons, not GPL (GNU Public License).
We've just updated the REBOL contest page clarifying that we will not accept contest entries that use GPL. Perhaps you are wondering why?
Generally, the GPL is an old software license that is hotly debated as to its precise meaning. Read GNU – A Virtual Communist Manifesto - II if you want to understand the problem better. There are many other such discussions -- too many for it to be considered a good license. Licenses need to be crystal clear.
Instead, if you want to use a license other than BSD or MIT, you may want to check out the Creative Commons Licenses for your scripts. We think they are a lot easier for both developers and users to understand.