Can I trust that my Geiger counter (GQ GMC 500) is in fact measuring radiation?

To put these low counts into perspective, I have watched quite a few YT vids of people exploring abandoned uranium mines. The background counts inside the mines were (to my mind) phenominal - like 50,000 CPM measured with a Radiacode RC103. Up close near the ore veins it could exceed 1,000,000 CPM! Miners used to work in those conditions. You would think they would all die within minutes at those levels but no, although not exactly healthy (and in years gone by some would suffer from cancers in later life) they were not dropping like flies. These levels are several orders of magnitude higher than anything you or I are likely to come across, even when handling our 'hottest' test sources. That's not to say you can be complacent about careful handling, especially alpha sources which may be ingested, but slight increases in background gamma activity are nothing to be concerned about. A gamma spectrometer in our case is used purely for academic interest to satisfy our curiosity. Professionals use very sensitive and expensive gamma spectrometers to measure things like food or land contamination but that kind of analysis is at another level.