The NuSTAR telescope has mirrors that reflect photons up to 79 keV, although only at glancing incidence (photon travelling close to parallel to the mirror surface). Astronomers typically regard energies below 100 keV as X-rays, but physicists regard photons emitted by atomic nuclei as gamma rays regardless of energy and some are in the range observed by NuSTAR, for example gamma rays emitted by decay of Titanium-44 in supernova remnants.

I don't know what the record is for photon energy reflected.

Edit: By contrast, the higher energy instrument on the Fermi gamma ray space telescope observes gamma rays from 20 MeV to 300 GeV, so at the high end that's over 3 million times as energetic as what NuSTAR observes. It does not use mirrors or lenses. Instead incoming gamma rays create electron-positron pairs and the telescope has a stack of detector layers that track their direction allowing the gamma ray direction to be determined fairly precisely, described in extreme detail by https://arxiv.org/abs/0902.1089 (scroll to end for the pictures)

INTEGRAL, working at lower energies from around 15 keV to 10 MeV, uses another approach, coded aperture masks. This is essentially the same idea as a pinhole camera, but with multiple pinholes and using computer process to unscramble the overlapping images.