The gm500 has two tubes right? I don't know how it chooses which one to use (probably changes after it gets saturated), but if the standart tube is something like a sbm-20 or j305 it probably picks up quite a bit of beta that goes through the ampoule, and the cpm to usv is most likely calibrated with the energy of cesium, so i think the actual doserate is lower that what it shows

Risks of low doses and low dose rates, such as from elevated natural background radiation exposures, appear not to exist or be lower than such risks that one assumes by applying the LNT model in the evaluation of epidemiological data. This and the unequivocal evidence of experimental findings of adaptive protection speak against the LNT hypothesis, which should be replaced by a model that takes into consideration that low doses can induce alterations in the physiologically individual balance between cancer causation and cancer prevention.

Source: Cancer Mortality Among People Living in Areas With Various Levels of Natural Background Radiation

I think it's indeed a bit higher than the background radiation in that altitude, but something like an electron beam irradiator can output as much as 11000 Gray per SECOND, the dose required for sterilization is pretty high. Also, in some places like Ramsar (Iran) and Guarapari (Brazil) the background radiation can be as high as 40uSv/h but that's pretty rare