This is an old unanswered question. gas.py does not (any more) contain that formula - it and other examples simply display Rs in kOhm.
The value measured is Rs. A problem is that we do not know R0, except that it varies hugely between sensors. And the relationship between Rs/R0 and ppm needs a log-log plot to make a straight line. But we can easily do a lot better that just displaying in kOhm