You can approximate the gradient by drawing a tangent.
But you will get a more accurate answer if you use a calculator in the following way.
Say you want to know the gradient of the curve y = x^5 (ie, y = x raised to the power 5) when x = 3.
That gradient is approximately
[(3 + very small number)^5 - 3^5] / (very small number)
The "very small number" has to be the same in both places.
For example, you could calculate
(3.00001^5 - 3^5)/(0.00001) = 405.003