the gradient of a curve is the same as the slope of the curve.
You can find it by taking the first derivative dy/dx of the relation.
If you do not know Calculus, and you want the slope of a curve at a given point,
find another point on the curve "extremely close" to your given point and find the slope between those two points the traditional way.
This will not be an exact answer, but very close to that exact answer.
e.g. find the slope to y = x^2 - 5x at the point (1,-4)
let x = 1.001, then y = -4.00299
so a point close to (1,-4) is (1.001,-4.00299)
then the slope = (-4.00299 - (-4))/(1.001 - 1) = -2.999
(using Calculus, the exact slope at (1,-4) is -3 )