I'm using the Arm A64 instruction set on a Raspberry Pi running Raspberry Pi 64-bit. Given the C code:

extern int x, y, z;

void addTwo(void)
{

  z = x + y;
}

gcc produces the assembly language:
        .arch armv8-a
        .file   "addTwoGlobal.c"
        .text
        .align  2
        .global addTwo
        .type   addTwo, %function
addTwo:
        adrp    x0, :got:x
        ldr     x0, [x0, #:got_lo12:x]
        ldr     w1, [x0]
        adrp    x0, :got:y
        ldr     x0, [x0, #:got_lo12:y]
        ldr     w0, [x0]
        add     w1, w1, w0
        adrp    x0, :got:z
        ldr     x0, [x0, #:got_lo12:z]
        str     w1, [x0]
        nop
        ret
        .size   addTwo, .-addTwo
        .ident  "GCC: (Debian 10.2.1-6) 10.2.1 20210110"
        .section        .note.GNU-stack,"",@progbits



I believe that the :got: and :got_lo12: modifiers tell the linker to use the address for x from the global offset table for the offset values in the
        adrp    x0, :got:x
        ldr     x0, [x0, #:got_lo12:x]


Instructions.

Using gdb, I can see that adrp loads the offset from the pc to the page number of x and that #:got_lo12:x is an immediate value that is the offset of x within that page.

But I cannot find where :got: and :got_lo12: are documented.