  We show how gauge coupling unification is successfully implemented through
non-supersymmetric grand unified theory, $SO(10)\times G_f (~G_f=S_4, SO(3)_f,
SU(3)_f)$, using low-scale flavor symmetric model of the type $SU(2)_L\times
U(1)_Y$ $ \times SU(3)_C \times S_4$ recently proposed by Hagedorn, Lindner,
and Mohapatra, while assigning matter-parity discrete symmetry for the dark
matter stability. For gauge coupling unification in the single-step breaking
case, we show that a color-octet fermion and a hyperchargeless weak-triplet
fermionic dark matter are the missing particles needed to complete its
MSSM-equivalent degrees of freedom. When these are included the model
automatically predicts the nonsupersymmetric grand unification with a scale
identical to the minimal supersymmetric standard model/grand unified theory
scale. We also find a two-step breaking model with Pati-Salam intermediate
symmetry where the dark matter and a low-mass color-octet scalar or the fermion
are signaled by grand unification. The proton-lifetime predictions are found to
be accessible to ongoing or planned searches in a number of models. We discuss
grand unified origin of the light fermionic triplet dark matter, the
color-octet fermion, and their phenomenology.
