We describe a scheme for generation of coherent γ-rays by backscattering intense visible laser light from a beam of partially stripped ions. The scheme is similar in principle to the proposed Gamma Factory at CERN, with the important difference that the scattering becomes coherent as a result of a collective instability which microbunches the ions. This instability is analogous to that which occurs in high-gain free electron lasers (FELs). The scheme potentially offers a route to a source of tuneable, coherent γ-rays, opening up a wide range of possible new applications and opportunities. We find that the parameter requirements for realization of coherent γ-ray generation regime are considerably more stringent than those proposed for the Gamma Factory, requiring significant increases in the pump laser intensity and possibly the ion beam current.