Fs and gs are in protected mode.
? segment registers
? segment selectors
? stack pointers
? register pointers

Segment registers

Explanation:

The initial purpose behind the segment registers was to enable a program to access many distinct (big) memory sections designed to be autonomous and part of a constant virtual store.

They don't have a processor-defined objective, but instead Operating system runs them for purpose. The GS register is used in Windows 64-bit to point to constructions defined by the operating scheme.   Operating system kernels usually use FS and GS to access thread-specific memory. In windows, thread-specific memory is managed using the GS register. To access cpu-specific memory, the linux kernel utilizes GS.