This might be an extremely dumb question, but... can a developer release a new version of his/her Q3A SDK mod under GPL?
If everything they used from the SDK (code which their mod is based on) is available in the GPL code base, and the mod has not used any resources from the original game (e.g. use a model with a new skin), plus all other resources are GPL compatible -- then basically "yes" they can relicense it under the GPL.
A precedent for this (not tested in court though) is that numerous DOOM source ports were originally under a quite restrictive license that the DOOM source code was released under, but later on Carmack re-released the DOOM code under the GPL, and those source ports relicensed themselves to the GPL.