Your jpeg seems to be a black and white image, you'd need at least a grayscale one to segment gray and white matter (also a neuroscientist may understand what you mean with gm and wm, but this question -same as the pic- is awfully generic).
You can then use a threshold to separate them in a single image. if the image has a gradient or a local bias a local adaptive thresholding method might be useful. There are tools to get the threshold automatically and to create probabilistic maps of the tissues. One example is ft_volumesegment from the fieldtrip toolbox.
