Hire a professional (Score 1)

The results you are going to get testing foodstuffs with a Gieger-Muller are going to be poor, almost certainly the levels of radiation in food are so low that the statistical fluctuations in the background count rate will render your results useless. You are definitely going to have to invest in some lead shielding to remove some of the background count to determine anything useful from your data. To do this properly I would recommend a more sensitive detector such as HPGe or an inorganic scintillator to actually determine the radioactive source content for proper analysis of the risks. I regret that someone is profiting horribly from the sale of GM tubes to people who are naturally scared for their health but have no idea how to use them and most importantly how to interpret the data they produce. Alpha particle emitters in foodstuffs are the most dangerous (tissue weighting factor x20 compared to gamma) but they will not be detected with a GM tube unless you place the source inside the tube itself. GM tubes with a thin window are quite good at detecting beta-particle emitters which are also harmful but it will be difficult to tell what source is actually emitting the radiation and the energy of the beta particles. The efficiency of a GM tube isn't that great with gamma radiation because it's so penetrating and gas naturally isn't very dense. As mentioned already calibration with a Cs-137 gamma source and a Sr-90 beta source by a should be carried out for a GM tube regularly, the efficiency of the detector can be calculated using some simple geometry calculations.