The sin of the angle is also the % gradient
.
Strictly speaking % gradient = 100% x
the tangent of the angle
You can see this if you look at your other comments
Altitude divided by Gradient = Distance
Altitude divided by Distance = Gradient
Gradient X Distance = Altitude.
In each case you are talking about the tangent of the angle not the sine.
But for small angles the tangent and the sine are almost the same, so in terms of getting the right answer it really doesn't matter much which you use.