In this paper we study the dependence of steady-state gain of a closed V-type atomic system and a closed Λ-type atomic system of lasing without inversion on the detuning of the coherence lasers under the condition that the corresponding parameters of the two atomic systems are equal. By the way of numerical simulation and comparison, we find that the curves of probe-laser gain of the two atomic systems versus the detuning of probe-laser have similar shapes but their values are different, which are related to the characteristics of atomic coherence and population distribution of the two atomic systems directly. In addition, we find that the driving laser deturning and the additional atomic coherence caused by spontaneously generated coherence can exert a great influence upon the probe-laser gain.