  For $\Lambda$ a selfinjective algebra, and $Q$ a finite quiver without
oriented cycles, the algebra $\Lambda Q$ is a Gorenstein algebra and the
category ${\rm Gproj}\Lambda Q$ of Gorenstein-projective $\Lambda Q$-modules is
a Frobenius category. For a sink $v$ of $Q$, we define a functor $F(v) :
\underline{\rm Gproj}\Lambda Q\to \underline{\rm Gproj}\Lambda Q(v)$ between
the stable categories modulo projectives, where $Q(v)$ is obtained from $Q$ by
changing the direction of each arrow ending in $v$. The functor is given by an
explicit construction on the level of objects and homomorphisms. Our main
result states that $F(v)$ is an equivalence of categories. In the case where
the underlying graph of $Q$ is a tree, we deduce that the stable category
$\underline{\rm Gproj}\Lambda Q$ does not depend on the orientation of $Q$.
Moreover, if $Q$ is a quiver of type $\mathbb A_3$ and $\Lambda=k[T]/(T^n)$ the
bounded polynomial algebra, we use the symmetry of the octahedron in the
octahedral axiom to verify that the composition of twelve reflections yields
the identity on objects.
