Originally Posted by italia458
For example: under FAR 25.121(b)(1) it requires 2.4% climb gradient from gear retraction to 400' above the Departure End of the Runway
Not exactly. 25.121(b) requires 2.4% gross climb gradient (for twins) "at the point where the landing gear is fully retracted" but without ground effect and at speed V2. The climb gradient reduces with altitude. Therefore at WAT-limit weight the average gross gradient between gear retracted and 400' will be somewhat less than 2.4%.
The take-off flight path requirements of 25.115 and the WAT-limit requirements of 25.121 are really quite separate requirements and should not be confused.