  Quantum discord is a measure of non-classical correlations, which are excess
correlations inherent in quantum states that cannot be accessed by classical
measurements. For multipartite states, the classically accessible correlations
can be defined by the mutual information of the multipartite measurement
outcomes. In general the quantum discord of an arbitrary quantum state involves
an optimisation of over the classical measurements which is hard to compute. In
this paper, we examine the quantum discord in the experimentally relevant case
when the quantum states are Gaussian and the measurements are restricted to
Gaussian measurements. We perform the optimisation over the measurements to
find the Gaussian discord of the bipartite EPR state and tripartite GHZ state
in the presence of different types of noise: uncorrelated noise, multiplicative
noise and correlated noise. We find that by adding uncorrelated noise and
multiplicative noise, the quantum discord always decreases. However, correlated
noise can either increase or decrease the quantum discord. We also find that
for low noise, the optimal classical measurements are single quadrature
measurements. As the noise increases, a dual quadrature measurement becomes
optimal.
