The fs and gs registers in modern OSes like Linux and Windows point to thread-specific and other OS-defined structures. Modifying the segment register is a protected instruction, so only the OS can set these up for you.
This question should help explain what exactly the point to: amd64 fs/gs registers in linux.
The actual value of the fs register isn't an address. It is a selector - an offset into the GDT, that describes what that segment can/cannot be used for. You cannot see what the values of the hidden fs base and limit registers are - they are internal CPU registers that are only updated by writing a new "selector" to fs (at which point the base/limit registers are updated from the GDT).
