Although this is not the best way of doing it, should also work fine. Please convert the original image to double before processing to avoid errors due to integer precision.
original = imread('grayscale.jpg');
original=double(original);
[x,y] = size(original);
hor_grad = zeros(x,y);
ver_grad = zeros(x,y);
for i=1:x
for j=2:y-1
hor_grad(i,j) = -1*original(i,j-1)+original(i,j+1);
end
end
for i=2:x-1
for j=1:y
ver_grad(i,j) = -1*original(i-1,j)+original(i+1,j);
end
end
figure;imshow(hor_grad,[])
figure;imshow(ver_grad,[])
