SO A ROAD WITH A GRADIENT OF 100% STICK VERTICALLY STRAIGHT UP IN THE AIR, RIGHT? No! 100% would mean “across 1, up 1”, so such a road would make an angle of $45$° with the horizontal. Pretty much nothing would be able to grip such a road – all the cars and people would simply slide down to the bottom. For those with a basic knowledge of trigonometry: the world’s steepest road makes an angle of $tan^{-1}(\frac{40}{100}) = 21.8$° with the horizontal.
AT GCSE: The gradient of a straight line is the $m$ in $y=mx+c$. It is simply a measure of how steep the line is. To find the gradient of a straight line ask yourself “across one (in the x-direction), up how much (in the y-direction)?“. So our steepest street in the image has a gradient of 0.4 (“across one up 0.4”).