GDT in 64-bit
The GDT is still present in 64-bit mode; a GDT must be defined, but is generally never changed or used for segmentation. The size of the register has been extended from 48 to 80 bits, and 64-bit selectors are always "flat" (thus, from 0x0000000000000000 to 0xFFFFFFFFFFFFFFFF).
If the System bit (4th bit of the Access field) is cleared, the size of the descriptor is 16 bytes instead of 8. This because, even though code/data segments are ignored, TSS are not, but the TSS pointer can be 64bit long and thus the descriptor needs more space to insert the higher dword of the TSS pointer.
64-bit versions of Windows forbid hooking of the GDT; attempting to do so will cause the machine to bug check.
Read more about this topic: Global Descriptor Table