Before going to reach at the conclusion first we have to understand the meaning of focusing.
Focusing is the process in which we bend any ray and concentrate it at a particular point or region ..A electromagnetic wave contains photons .Whenever the ray is passed through any focusing devices just like collimator,lens,mirror etc. the photons are passed through these materials and don't interact with the materials of these devices.Hence they are concentrated at a particular point.
Focusing will be difficult if the ray is very energetic .In this case the photons of the ray will interact with device and will not bend just like normal visible light light rays.
Gamma ray,X-ray waves are very energetic.Now special devices are designed for X -ray also.But gamma ray has high frequency in the eletromagnetic spectrum.So it interacts heavily with the matter and destroys itself or its energy is altered in a greater amount. So it is very difficult to focus gamma ray.
