I mean, this isn't THAT surprising. Photosynthesis after all is just radiosynthesis of electromagnetic radiation in the visible or near visible spectrum. Gamma radiation is the same phenomenon, just with a far higher frequency and enough energy to ionize molecules.
The chemical process obviously has to differ considering gamma radiation has enough energy to knock off electrons, but once you deal with that, it's energy ripe for the taking. I'm not shocked that life finds a way to harness that energy where abundant.
In fact, had life come about on an Earth with a weaker magnetic field, it may have relied more on gamma radiation than visible light, especially considering the larger potential amount of consumable energy present in gamma rays.