In our recent work, Phys. Rev. Lett. 108, 086402 (2012), we showed the intriguing properties of the ferromagnetic state in the Kondo lattice model. We showed that at zero temperature, the minority conduction electrons form an insulating state, while the majority conduction electrons form a metallic state. Furthermore, we showed that the gap width in the spectral function of the minority conduction electrons obeys a Kondo-temperature-like form, similar to exp(-1/J), in the coupling strength. We explained that the gap is due to the commensurability between the local spins and the conduction electrons arranging themselves in a way that the number of minority electrons per lattice site adds up to unity. Here, we show how the gap and the commensurability are formed when the system enters the ferromagnetic phase when the temperature is lowered. We also clarify that this commensurable situation is not due to a special choice of system parameters or to an adjustment of the filling of the lattice, but is general for a ferromagnetic state in the Kondo lattice model.