  Necessary and sufficient conditions for arbitrary multimode (pure or mixed)
Gaussian states to be equivalent under incoherent Gaussian operations are
derived. We show that two Gaussian states are incoherent equivalence if and
only if they are related by incoherent unitaries. This builds the counterpart
of the celebrated result that two pure entangled states are equivalent under
LOCC if and only if they are related by local unitaries. Furthermore,
incoherent equivalence of Gaussian states is equivalent to frozen coherence
[Phys. Rev. Lett. \textbf{114}, 210401 (2015)]. Basing this as foundation, we
find all measures of coherence are frozen for an initial Gaussian state under
strongly incoherent Gaussian operations if and only if the relative entropy
measure of coherence is frozen for the state. This gives an entropy-based
dynamical condition in which the coherence of an open quantum system is totally
unaffected by noise.
