  In high energy heavy-ion collisions, the degrees of freedom at the very early
stage can be effectively represented by strong classical gluonic fields within
the Color Glass Condensate framework. As the system expands, the strong gluonic
fields eventually become weak such that an equivalent description using the
gluonic particle degrees of freedom starts to become valid. We revisit the
spectrum of these gluonic particles by solving the classical Yang-Mills
equations semi-analytically with the solutions having the form of power series
expansions in the proper time. We propose a different formula for the gluon
spectrum which is consistent with energy density during the whole time
evolution. We find that the chromo-electric fields have larger contributions to
the gluon spectrum than the chromo-magnetic fields do. Furthermore, the large
momentum modes take less time to reach the weak-field regime while smaller
momentum modes take more time. The resulting functional form of the gluon
spectrum is exponential in nature and the spectrum is close to a thermal
distrubtion with effective temperatures around $0.6$ to $0.9\, Q_s$ late in the
Glasma evolution. The sensitiveness of the gluon spectrum to the infrared and
the ultraviolet cut-offs are discussed.
