Generating a 1-bit GIF with ImageIO

gistfile1.m

- (void) generate1BitGIF
{
    UIImage* image = [ UIImage imageNamed: @"TestImage.png" ];

    NSFileManager* fm = [ NSFileManager defaultManager ];
    NSURL* destDir = [ fm URLForDirectory: NSDocumentDirectory 
                                 inDomain: NSUserDomainMask
                        appropriateForURL: nil
                                   create: YES
                                    error: nil ];
    NSURL* destUrl = [ destDir URLByAppendingPathComponent: @"test.gif" ];
    
    CGImageDestinationRef dst = CGImageDestinationCreateWithURL( (CFURLRef) destUrl, kUTTypeGIF, 1, NULL );
    
    const uint8_t colorTable[ 6 ] = { 0, 0, 0, 255, 255, 255 };
    NSData* colorTableData = [ NSData dataWithBytes: colorTable length: 6 ];
    
    NSDictionary* gifProps = [ NSDictionary dictionaryWithObject: colorTableData 
                                                          forKey: (NSString*) kCGImagePropertyGIFImageColorMap ];
    
    NSDictionary* imgProps = [ NSDictionary dictionaryWithObject: gifProps 
                                                          forKey: (NSString*) kCGImagePropertyGIFDictionary ];
    
    CGImageDestinationAddImage( dst, image.CGImage, (CFDictionaryRef) imgProps );
    CGImageDestinationFinalize( dst );
    CFRelease( dst );
}

What does this do? Tried using it, colorTable doesn't seem to do anything?