  In spite of their outstanding experimental relevance, Gaussian operations in
continuous-variable quantum systems are subjected to fundamental limitations,
as it is known that general resources cannot be distilled within the Gaussian
paradigm. We show that these limitations can be overcome by considering a
collaborative setting where one party increases the amount of local resource
with the aid of another party, whose operations are assumed to be Gaussian but
are otherwise unrestricted; the two parties can only communicate classically.
We show that in single-shot scenarios, unlike in the well-known case of
entanglement theory, two-way classical communication does not lead to any
improvement over one-way classical communication from the aiding party to the
aided party. We then provide a concise general expression for the Gaussian
resource of assistance, i.e., the maximum amount of resource that can be
obtained when the aiding party holds a purification of the aided party's state,
as measured by a general monotone. To demonstrate its usefulness, we apply our
result to two important kinds of resources, squeezing and entanglement, and
find some simple analytic solutions. In the case of entanglement theory, we are
able to find general upper bounds on the regularized Gaussian entanglement of
assistance, and to establish additivity for tensor powers of thermal states.
This allows us to draw a quantitative and enlightening comparison with the
performance of assisted entanglement distillation in the non-Gaussian setting.
On the technical side, we develop some variational expressions to handle
functions of symplectic eigenvalues that may be of independent interest. Our
results suggest further potential for Gaussian operations to play a major role
in practical quantum information processing protocols.
