Is it allowed by GPL, to make actually a copy of a map from quake3?
With copy I mean, decompile it, build all new (decompiled map used for distances and such), use different textures and models.
I mean q3dm6, I thought that is illegal, but past few days I have seen a version for UT3, and a version for Doom3, so.. I think that is ok?
I love that map allot, and I would like to copy it. I have seen q3dm6ish, but its to different.
Nope, you may not decompile a map (or any other media) and make copies of it if it is copyrighted material, unless you have the authors specific permission (preferably in writing) to do so.
However, what you may, and many do, is to use landmarks as reference points and then create your own map, based on your own measurements. By decompiling something, even as trivial as a map, you could be sentenced to jail as it is an infringement on copyright in the same way as reverse engineer source code of a program/engine would be.