For each pixel, the value for the red, green and blue channels should be their averages. Like this:
int red = pixel.R;
int green = pixel.G;
int blue = pixel.B;
pixel.R = pixel.G = pixel.B = (red + green + blue) / 3;
Since in your case the pixel colors seem to be stored in an array rather than in properties, your code could end up looking like:
int red = pixel[0];
int green = pixel[1];
int blue = pixel[2];
pixel[0] = pixel[1] = pixel[2] = (red + green + blue) / 3;
The general idea is that when you have a gray scale image, each pixel's color measures only the intensity of light at that point - and the way we perceive that is the average of the intensity for each color channel.