mm writes:

AFAIC gases don't weigh
anything.

Of course they weigh something.

Some are denser than air and sink.
For example CO2. Remember that village smothered in
Africa. That was CO2 rising from a lake then moving along
the ground.

Some are lighter than air and rise.
Helium is an everyday example.

Some like Nitrogen and Oxygen are about the same
as the mixture of gases we call air and just disperse.
If you weighed them on an everyday scale the scale would
say zero, but they have mass and are attracted to the Earth
by gravity.