The correct equation is
% Climb gradient = approximately 100% ( (Thrust - Drag)/Weight)
Lift is less than weight in a steady climb, but for small climb angles the difference is very small. If we ignore this small difference we can say that lift = weight.
We can now resate the above equation as
% Climb Gradient = 100% ((Thrust/Weight) - (Drag/Lift))
But Drag/Lift is just 1 / Lift to Drag ratio
So we can restate the equation as
% Climb Gradient = 100% ((Thrust/Weight) - (1/ Lift to Drag ratio))
In this question we get
% Climb Gradient = 100% ((120000 N / 500000 N) - (1/12)
% Climb Gradient = 15.67%
All of the above is of course an approximation based on the simplfying assumption that lift = weight.
But it gives us a nice easy equation which bypasses the need to calculate the drag.
% Climb Gradient = 100% ((Thrust/Weight) - (1/Lift to Drag Ratio))