[Old Pedant]

Missing the apostrophes around the PHP variables. Why did you think you could omit them in the UPDATE section if you needed them in the INSERT section???

But I'd do this:

// ensure no sql injection and also put the apostrophes around each value
// so that you only have to do this once!
function clean( $val )
{
    return "'" . mysql_real_escape_string( $val ) . "'"
}
// do it for each value:
$gamename = clean($gamename);
$cleangc = clean($cleangc);
$region = clean($region);
$mode = clean($mode);
$vmc = clean($vmc);
$smb = clean($smb);
$hdd = clean($hdd);
$usb = clean($usb);
$notes = clean($notes);
$comp = clean($comp);

$sql = "INSERT INTO $tbl_name(gamename, gamecode, region, mode, vmc, smb, hdd, usb, notes, comp)
       VALUES($gamename,$cleangc,$region,$mode,$vmc,$smb,$hdd,$usb,$notes,$comp) "
     ON DUPLICATE KEY UPDATE 
     gamename = IF(gamename = '', $gamename, gamename),
     region = IF (gamename = '', $region, region),
     mode = IF(gamename = '', $mode, mode),
     vmc = IF(gamename = '', $vmc, vmc), 
     smb = IF(gamename = '', $smb, smb), 
     hdd = IF(gamename = '', $hdd, hdd), 
     usb = IF(gamename = '', $usb, usb), 
     notes = IF(gamename = '', $notes, notes), 
     comp = IF(gamename = '', $comp, comp)";