The GSL is quite different. Roughly speaking, it doesn't license the reproduction of any text in which WotC holds the copyright. It is basically a trademark (and affiliated product identity) licence, and in that respect therefore more comparable to the D20 licence (and like the D20 licence, and like most trademark licences, it is not in perpetuity). Once WotC no longer wishes to use the trademarks in question, I'm pretty sure it will pull the GSL, as it won't want others to be trading under those marks either. But this will have only modest implications for any trying to clone 4e - either with or without the GSL, the issue with cloning 4e is not mostly a trademark issue (though that could perhaps come up if you copied the WotC stat blocks very closely) but rather a copyright issue.
@Frylock wrote some good blogs about this a year or two ago.
Given the number of "Unauthorised strategy guides" that were produced for computer games in the 90s, I'm not sure that even pulling the GSL would do a hell of a lot to people who want to produce material for 4e. (And yes, I've been vaguely looking into an idea for this). Am I wrong?
Goodman Games did not use the GSL for its 4e products...at least the 8 or 10 modules I bought.
Who else has Goodman Games 4E products and could let us know how they can go GSL-less? Or are we just agreeing that anything made as system-neutral is unofficial 4E support (which means there is tons of unofficial 4E support and always will be)?
http://www.goodman-games.com/4eproducts.html
As I posted upthread, and just above this one, there is no problem with producing 4e support provided that you respect WotC's intellectual property rights.Hrm, am I missing something or is there a contradiction here?