Most (maybe all?) CGMs need to be calibrated regularly with finger pricks. In fact in the case of both the Dexcom G5 (haven't known anyone on the G6) and Medtronic Guardian sensors they can occasionally sort of derp out and ask for multiple calibrations in a row for utterly mysterious reasons. I just asked and my partner says the most times she has had to test to get a Guardian sensor back to normal was seven times in two hours.

Of course this is anecdotal, but the point is that your mileage may vary.

Apparently there's a problem with the Guardian transmitter that causes the multiple calibration requests. Minimed is taking orders for free replacements: https://info.medtronicdiabetes.com/bgcheck

I switched to the Dexcom G6 though. It's amazing. I'm using it with open source closed loop pump software for iOS called Loop. There's also a Linux based solution called OpenAPS.

Freestyle Libre isn't strictly a CGM, but it doesn't need calibration.

Dexcom 6 doesn't need to be calibrated anymore. Well, it still needs the first calibration when you insert it, but none other than that. Lasts for some 2-3 weeks before sensor needs replacing.

The Dexcom G6 system does not require any calibration, insertion is remarkably pain-free, and it lasts for 10 days. I've had it for a few months (after a couple of years of using the G5 system) and I can't imagine living without it.

Yeah I like it too, but it still asks me for the 2 calibrations right after insertion.

To be fair, that could be just xDrip+ that wants calibrations.