you need a known source such as Cs-137 with certificate and proper set-up to calibrate the GM kit, otherwise the reading is meaningless.
Indeed. The calibration is a complex issue and it depends on the purpose for which you are doing it in the first place. I think that
this kind of kits should not display any purported measurement unit like µS/h, just counts per minute. So you know that a piece of pottery gives a much higher CPM count than the usual background level you see, but there's no potentially misleading µS/h value.
What I mean is that the detector works (not that using a Geiger tube is rocket science), it shows a roughly meaninfgful value for background radiation, and it indeed reacts when you place a radioactive source (beta or gamma) close to the tube.
The tube included with the circuit does not detect alpha particles, but they give you instructions to try different tubes. I have purchased a SBM-9 which should detect alphas as well. I'll try it one of these days.