lenth(f) has no obvious relationship to any of the rest of the code. Is f shorter than dev is so you are only wanting to do a subset of dev ? If you are wanting all of dev then use the number of elements in dev, not the number in f.
You initialize Grad to size(x) but you only iterate i as the length(x) not as the number of elements in x.
It is not obvious that length(x) or size(x) is related to the size of GradW or dev.
x = randi(9, 1, size(GradW,1))
f = randi(9, 1, size(GradW,2))
answer1 = reshape(2*GradW * dev(:), size(x))
Grad(i) = Grad(i) + 2*GradW(i,j).*dev(j);
You can see the probable simple formula at answer1, but because your sizes are not related to the variables you calculate on, we cannot be sure.
It seems odd to want to force the answer to be the shape of x; it would make more sense to let it be a column vector (in which case skip the reshape() of answer1)