Excuse me, it was some kind of lapsus :)

I tried to install
meta-pkgs/kde3,
no meta-pkgs/xorg.

Anyway, libgthread is (as long as I know) a thread library (I do not
know if it is a full implementation or a wrapper of some lower level
library) included with glib.

I am going to try the weekend with your suggestions. Thanks a lot.