We present calculations of the linear optical absorption/gain in ZnSe including the relevant many-body effects which comply with a correct description of position and linewidth of the exciton and of the transition from gain to absorption at the chemical potential. The influence of an excitonic fraction is included considering the chemical equilibrium of bound e-h pairs and free carriers by a T-matrix approach. For temperatures from 40 upto 200 K;, we found that increasing the excitation the chemical potential comes close to the excitonic absorption peak but does not cross it, as long as the latter survives. Moreover, there is only a minor fraction of excitons under gain conditions. We deduce, that the gain mechanism is governed by many-body correlations of a strongly Coulomb-correlated electron-hole plasma.