This would mean that the gas has a total mass of the initial body (at max), because gas is not simply generated but results of a conversion process.
If certain bacteria cause this it may be possible to generate more gas than body mass but than the additional mass and energy has to come from something else than the aliens (like said, bacteria, air, water or something like this).
Aside from that, if you calculate the mass of the aliens to lets say 200kg
and if we assume the gas is only generated in this mass. For full earths atmosphere (lets say the gas could spread up into the Thermosphere (250km) this would mean you would have to cover a volume of
VolumeEarthMass(4/3*pi*radius(~6378.1 km)^3=~1.083.206.916.845 km3)=
(I hope i did not mess up the original numbers in here but my calculation/result should be quite right for an estimate)
So you would have to cover this huge volume with 200kg worth of mass.
If we additionally assume the gas is similar to methane (which is usually generated on most composition processes) one kg of the gas is 1,47017 m3 -> 1,47017*10^-9 km3 -> 200kg of gas are 2.94034E-7 km3
And the Volume of the Atmosphere divided by the Volume of the Gas would result in a density of 2.1549544*10^-20 which is 0.0000000000000000021549
Meaning the chance to stumble into a gas particle is very very low (I won't calculate this too as it is late and as I am not sure how exactly).
You could now calculate how much gas they need to generate in mass to make it at least possible for many humans to breath the gas, or you scale the atmosphere (spreading height of the gas) down until the numbers get better.
Tinker with the numbers, or take s bit of exaggeration out of her saying so it only relates to a smaller area of earth.
Or get more aliens to earth...
I hope the calculations and assumptions helped and were at least nearly correct.
12 Years of German Education System
Ignoring this post I am curious about your story (trial reading application) ;)