__seg_gs specifier in g++

["J.W. Jagersma" <jwjagersma@xxxxxxxxx>]

I'm trying to use the __seg_gs address space specifier in g++, but it
keeps telling me "__seg_gs does not name a type". Even the following
short snippet fails to compile:

int main()
{
#ifdef __SEG_GS
    __seg_gs int* i;
#endif
}

Am I doing something wrong here? Any help would be appreciated.