The 2.4% is a legislated figure which applies to all air transport twins above 5700kg. All this means is that, for the takeoff weight altitude and temperature, a 2.4% gross climb gradient (1.6% nett) must be achievable on one engine in the second segment IF there are no other limitations in the takeoff path.
Some runways or departure procedures may specify that a greater climb gradient is required - hence the need for the graphs in the AFM or some other method such as computer derived tables. The operator and pilot are required to ensure that whatever climb gradient it takes, can be achieved - 1.6/ 2.4% merely being the minimum allowable if there are no other limiting factors.
I appreciate that this may be self-evident to most readers of this section, but the preceding post may be read by the uninitiated to imply that the B737 is somehow guaranteed to climb at 2.4% when it most certainly is not, and won't, it you load it up enough.