If its of any help I use a quick rule of thumb for such problems. It goes like this: If for missed approach I must achieve 2.5% climb gradient all you do is multiply your ground speed x 2.5 to obtain required rate of climb. In this case lets assume my ground speed to be 120kts so 120 x 2.5 = 300fpm required. Simple! Rules of thumb are on the go in seconds simple calculations. Not only for gradients of climb but also for degrees of climb. Suggest to learn more about it
here only if you care.