The paper reports on the effects of Gamma radiation to single-photon avalanche diodes (SPADs). To the best of our knowledge, this is the first study ever conducted on SPADs fabricated in CMOS technology. A typical CMOS SPAD consists of a pn junction reverse biased above breakdown, so as to operate in Geiger mode. The multiplication region is confined to the planar zone of the junction by means of a guard ring that reduces the electric field in the periphery. Gamma radiation is known to cause crystallographic defects in semiconductor substrates. Such defects may in turn create new carrier recombination-generation trapping centers in the multiplication region and elsewhere, thus increasing thermally generated dark counts and afterpulsing. In this study, we show that a dose of 30krad results in an average increase of 10% of dark count rate and afterpulse probability, while timing jitter and other performance measures, typically independent from trap levels, remain largely unchanged.