You can only concatenate vectors horizontally if they have the same number of rows. But here you have 256 and 18 rows. sp this does not work. You can add zeros to g2 to blow it up to a 256 x 1 vector using
g2(256) = 0;
before you use
V = [ g1 g2 V ];
Another option would be to use cell arrays. The elements can have different sizes:
V = {g1, g2, V};
