This will not answer your question but i can tell you how I'm sure of making a climb gradient.
In my company, (i'm sure a similar procedure is used in most airlines) we use RTOW tables to compute the flex / assumed temperature. These tables will guide you to take the minimum power setting to meet all regulatory requirements as well as performance factors, such as Field limit, obstacle limit, climb limit and a few others.
Knowing that the assumed temperature, (or full power as the case may be) was computed correctly, I know that we can meet the climb gradient for that particular departure under the ambient conditions.