Focusing gamma rays seems out of the question since their wavelengths (less than 0.01 angstrom) are smaller than the distance between atoms in solids. Thus, they must de detected individually using techniques borrowed from elementary-particle physics.
Giovanni F. Bignami, Sky & Telescope, October 1985
 

Higher-energy X-ray photons can pas through a lens, but since they undergo no significant deflection, no focusing can take place.
Gerald K. Skinner, Scientific American, August 1988
 

Whereas optical photons are coherently scattered by the lens and thus can be focused directly into the image plane, gamma-rays can not be focused. They are scattered incoherently and the direction of the scattered electrons are lost.
von Ballmoos et al., Astron. Astrophys. 221, 396, 1989