A 4e clone would not use the GSL. It would use the OGL, which is not revokable or amendable by Hasbro.
Creating a 4e clone without infringing copyright would be tricky, though. I think WoTC deliberately made this hard to do, by eg renaming monsters and creating a lot of new, copyright-protected 'fluff', as well as systems that depart quite radically from 0e-3e. When taking the raw, non-protectable, game mechanics* from 4e, you would need to be very careful not to take any of the copyrightable expression along. All your 'fluff' would need to come from the SRD, or be original.
I teach copyright law for a living and I could probably do it correctly, were I willing & able to put in the large amount of effort it would require. I think for most people it would be pretty risky, though**.
*By contrast it is fine to call eg Healing Surges 'Healing Surges'. A 2-word term like that is not copyright protected. And changing the term won't shield you from liability if your description of how they work is itself a derivative work/adaptation.
**As it is, the guys who did OSRIC did a very good job avoiding any possibility of a (c)-claim; some of the other existing retro-clones look a bit shakier, though.