Once you know, its easy, but at first the guesswork starts: What in heavens is “gsm”? And how much is 20mil? What is a ‘mil’ anyway?
A google/bing search shows ‘Global System for Mobile Communications.’ Hm, no, apparently not that GSM is meant here. But I see a search topic “is gsm 2g or 3g”, looks promising. Maybe this is a flex weight of 2-3 grams? No! Its the topis “is GSM a 2nd or 3rd generation.” But on the very bottom of the page I see a search topic “gsm paper”: Gotcha! Its g/m^2, gram per square meter. Ok, so we are in the SI unit territory here, but with a trade common designation. In SI ‘gsm’ means literally “gram second meter” and is kind of a nonsense.
Now the ‘mil’ is however apparently 1/1000 of an inch, a non SI unit of measure. Punch in “what is 20mil in mm” and apparently thousands of people do that. Its 0.508mm. Divide mil by 39.37 to obtain millimeter.
Thus the paper thickness is approx 0.5mm, now finally I can picture it. Its thick!
Paper is not alone in this issue of confusing measures. We have a similar goulash of units in the photography on every step: focal length is in mm, but the monitor sizes are in inch-diagonal. Sensor sizes are even worse, its kind of approximately 1.5 * diagonal in inches, the infamous 4/3rd inch comes to mind… Unless consumer laws will require to show on the package standard measures, equal to all, we have to live with it. Imagine how helpless one would be without the web search !