Don't get confused!
Use the I formation I provided to find the relevant paper thickness, then find (in terms of thickness) a corresponding US paper.
So, referring to the chart....
US/Metric Paper Conversions
120 grn = 065 of an inch. The closest equivalent thickness can be found in US 36 lb BOND or LEDGER. So that's what you need. Simple!
If you read the information I provided you will see that thickness in US paper is NOT constant. But providing the thickness is close you will be fine. Check with a caliper just to be sure, and then go up or down a size if you have to. You will always use the paper anyway!
And as I said before, for Johan's model's use the paper he recommends - a fraction thicker is fine. I have made many of his models (some more then once), and where I have used other paper sizes, have had issues related to shaping the model or in terms of its strength.