PIC12F683 or PIC16F688 assembly or C blinking LED example with XC8 compiler, MPASM or gputils
Apart from missing the end bracket (copy/paste missed the last character?) your code works for me. However there are a couple of omissions that might make you think it doesn't:-


#define _XTAL_FREQ 8000000 tells the compiler what frequency the PIC should be running at, but does not actually set it. On start up the 12F683 runs at 4MHz. To change it to 8MHz you must set OSCCONto 0x70 (bits 6-4 all '1').
Without a delay after changing the I/O pin's state it will toggle very fast (with an unequal up/down ratio). XC8 has built-in functions __delay_us() and __delay_ms() for creating long delays.    


Here is your code modified to run the 12F683 at 8MHz with GP0 toggling at 1Hz.

void main(void) {
  OSCCON = 0x70; // switch to 8MHz system clock
  GPIO = 0x00;
  ANSEL = 0x00;
  TRISIO = 0x00;
  ADCON0 = 0x00;
  CMCON0 = 7;
    while(1)
    {
        GP0 = 1;
        __delay_ms(500); // wait 500 milliseconds
        GP0 = 0;
        __delay_ms(500); // wait 500 milliseconds
    }
}