You have the formula correct, however you need to know the Lift/Drag ratio calculation.
This is 1/12 x 100, which will give you - the number 8.333
Formula is correct, but can be confusing to do the calculation. The lift/drag ratio will always be as above, example 1/8 x 100 for 8 lift drag ratio.
What you do which I found better is:
(120.000N / 500.000)( x 100)) - 1/12 x 100 = Climb gradient
(0.24) x 100 - (0.083 x 100) - 15.67%
Better to write:
Climb gradient = Thrust / Weight x 100% - (Lift / Drag) x100%
Weight is mass x gravity, 50.000 kg x 10 (not 100)
so the first part is actually:
(Thrust / (Mass x Gravity)) (x 100%)
Remember Weight is not 50.000 kg, Weight is 500.000N in this example.
Mass and Weight are not the same.