  We present an implementation of the $GW$ space-time approach that allows
cubic-scaling all-electron calculations with standard Gaussian basis sets
without exploiting any localization nor sparsity considerations. The
independent-electron susceptibility is constructed in a time representation
over a non-uniform distribution of real-space locations $\lbrace {\bf r}_k
\rbrace$ optimized within a separable resolution-of-the-identity framework to
reproduce standard Coulomb-fitting calculations with meV accuracy. The
compactness of the obtained $\lbrace {\bf r}_k \rbrace$ distribution leads to a
crossover with the standard Coulomb-fitting scheme for system sizes below a few
hundred electrons. The needed analytic continuation follows a recent approach
that requires the continuation of the screened Coulomb potential rather than
the much more structured self-energy. The present scheme is benchmarked over
large molecular sets and scaling properties are demonstrated on a family of
defected hexagonal boron-nitride flakes containing up to 6000 electrons.
