This doesn't mean much, because almost any licence is not the GPL (specifically - any licence that is not the GPL, including licences such as the BSD licence). There is a message in some ljcom files describing how and why the files are not GPL:
# LiveJournal.com-specific library
# This file is NOT licensed under the GPL. As with everything in the
# "ljcom" CVS repository, this file is the property of Danga
# Interactive and is made available to the public only as a reference
# as to the best way to modify/extend the base LiveJournal server code
# (which is licensed under the GPL).
# Feel free to read and learn from things in "ljcom", but don't use
# our schemes because we don't want your site looking like
# LiveJournal.com (our logo and site scheme are our identity and we
# don't want to confuse users)
However, none of this actually describes the licence under which the files are released. In my experience with LiveJournal it seems that TPTB wish to have site-specific schemes (ljcom) stay at LiveJournal.com. However, this information is not freely available.
One solution to this would be to include a README in the root of the ljcom CVS describing exactly the licence of the files included in the repository. Additionally, I might recommend that a blurb be added to styles like Opal (which are what LiveJournal wishes to keep most, in order to have a seperate identity) that describes possible uses.
Granted, there really isn't any way to prevent other sites from using the code if the source is viewable. However, by providing provisions, LiveJournal can at least encourage behavior that fits with the expectations that the LiveJournal powers that be wish to encourage.
Suggested text might be something like:
"This style was written for LiveJournal.com. Although the source is viewable, this style is not to be copied to other sites without permission." Or something like that, similar to the warning included in the dystopia source.