If such blowing from below does exist, how can you certain that gravity produce air dense on the ground?

Measurements of it and simple logic.

There are quite a few ways to explain it. One is like this:

Any layer of air needs to support all the weight of the air above.

As you go down through the air, more and more weight is added, and thus the pressure increases.

You can also do some simple math. If you have a column, with an cross sectional area of A, and a height of h, with the pressure pushing down at the top of P, the column is made of air with a density of p:

The volume of the column is A*h.

The mass is p*A*h.

The weight is g*p*A*h.

The force due to the pressure at the top is P*A.

The force at the bottom is P*A + g*p*A*h

The pressure at the bottom is (P*A + g*p*A*h)/A=P+g*p*h

And this pressure is hydrostatic. If it wasn't constant at the bottom, the higher pressure would push outwards.

So if you have 2 columns of air, side by side, where they each have a different density, the greater density air will push the lower density air out of the way, pushing it up so the more dense air falls and the less dense air rises.

This is also why and other fluids are self-leveling.

Try again

Why? You are the one who needs to try again.

You still haven't got your candle flame without gas around it, nor have you got a flame in 0g.