Change following lines in you code function getAvgGradient().
1. blksize is a 1x2 array. zeros() can only accept a scalar as input
GxData = zeros(blksize(1)^2, 1);
GyData = zeros(blksize(1)^2, 1);
GsxData = zeros(blksize(1)^2, 1);
GsyData = zeros(blksize(1)^2, 1);
2. In MATLAB, the index start from 1
for row = 1:size(mag, 1) % change from 0 to 1
for col = 1: size(mag, 2) % change from 0 to 1
