That’s a grading-incidence mirror, and it’s used for X-rays. The Chandra and NuSTAR space telescopes are good examples of their use. They’re very low efficiency and heavy, though, and that would get even worse for gamma rays, where the incidence angle would have to be even smaller.

In practice a gamma ray telescope uses collimators (basically blocking all light other than that which is coming from straight on) or coded-aperture masks, which are kind of like pinhole cameras. Neither of those involve reflecting gamma rays though.