gdt.h:
#include <types.h>

struct gdt_seg_entry {
	uint16_t limit;
	uint16_t offset1;
	uint8_t offset2;
	uint8_t access;
	uint8_t limitflags;
	uint8_t offset3;
} __attribute__((packed));
typedef struct gdt_seg_entry gdt_seg_entry;

struct gdt_tss_entry {
	uint16_t limit;
	uint16_t offset1;
	uint8_t offset2;
	uint8_t access;
	uint8_t limitflags;
	uint8_t offset3;
	uint32_t offset4;
	uint32_t reserved;
} __attribute__((packed));
typedef struct gdt_tss_entry gdt_tss_entry;

struct gdt_p {
	uint16_t size;
	uint64_t offset;
} __attribute__((packed));
typedef struct gdt_p gdt_p;

void add_gdt_entry(uint32_t num, uint32_t offset, uint32_t limit, uint8_t access, uint8_t flags);
void add_gdt_tss(uint32_t num, uint64_t offset, uint32_t limit, uint8_t access, uint8_t flags);
void load_gdt(gdt_p* pointer);
void init_gdt(void);

gdt.c:
#include <gdt.h>
#include <tss.h>

gdt_seg_entry gdt[6];
gdt_p gdt_pointer;

void add_gdt_entry(uint32_t num, uint32_t offset, uint32_t limit, uint8_t access, uint8_t flags)
{
	gdt[num].offset1 = offset & 0xffff;
	gdt[num].offset2 = (offset >> 16) & 0xff;
	gdt[num].offset3 = (uint8_t)(offset >> 24) & 0xff;
	gdt[num].limit = limit & 0xffff;
	gdt[num].limitflags = (limit >> 16) & 0xf;
	gdt[num].access = access;
	gdt[num].limitflags = flags << 4;
}

void add_gdt_tss(uint32_t num, uint64_t offset, uint32_t limit, uint8_t access, uint8_t flags)
{
	add_gdt_entry(num, offset & 0xffffffff, limit, access, flags);
}

void load_gdt(gdt_p* pointer)
{
	__asm__ volatile ("lgdt (%0);" : : "r"(pointer) : );
}

void init_gdt()
{
	gdt_pointer.offset = (uint64_t)&gdt;
	gdt_pointer.size = sizeof(gdt) - 1;

	add_gdt_entry(0, 0, 0, 0, 0);
	add_gdt_entry(1, 0, 0xfffff, 0x9a, 0xa);
	add_gdt_entry(2, 0, 0xfffff, 0x92, 0xc);
	add_gdt_entry(3, 0, 0xfffff, 0xfa, 0xa);
	add_gdt_entry(4, 0, 0xfffff, 0xf2, 0xc);
	add_gdt_tss(5, (uint64_t)&tss, sizeof(tss_type), 0x89, 0);

	load_gdt(&gdt_pointer);
}