There's another parameter worth considering - how efficient is it to convert sunlight vs. gamma radiation into biologically usable energy.

What if for some reason gamma radiation changes the equilibrium constants for ADP --> ATP?

Another hypothesis to test would be if the radiation is being used as a catalyst somehow.

E.g. Could be denaturing something else, unlocking a previously inaccessible energy source. Possibly some radiochemistry creating a new food source for the fungus too.