  Similarly to the action of $Out(F_N)$ on Outer Space, the outer automorphism
group of a Generalized Baumslag Solitar group acts on a deformation space
endowed with the Lipschitz metric and the action of any fully irreducible
automorphism with a train track representative is hyperbolic. Inspired by
previous work by Algom-Kfir, we prove that the axis of such an automorphism is
strongly contracting.
