That's because you're only asking for a single output (by default). If you want both outputs, you must include two outputs in your call to the function.
[retx, rety] = gradlandscape(3, 3)
If the intention of the function was to produce a single output that is a 1 x 2 vector instead of two scalar outputs, then you'll need to change the function like this:
function retxy = gradlandscape(x, y)
h = 0.1;
retxy(1) = (landscape(x+h, y)-landscape(x-h, y))/(2*h);
retxy(2) = (landscape(x, y+h)-landscape(x, y-h))/(2*h);