What equipment do you need to find the density of a gas?
Of course, there are state functions (for instance the Ideal Gas equation), that can express density as a function of temperature, pressure, and volume, but for experimental measurement we must take the quotient...........
And thus you could take the tare value of a glass bulb of known volume, fill it with gas at (say) atmospheric pressure, and then remeasure the mass of the bulb..... The measurement is not straightforward in that you have to pfaff about with gases, and you may have to evacuate the glass bulb, and fill it with gas at a known pressure (typically atmospheric)..........or at least flush thru the gas to be measured at atmospheric pressure.
For a simple example with a condensed phase, see here.