If you create a vacuum, and drop some objects inside said vacuum all objects irrespective of their density will still fall at 9.8 m/s^2 due to the pull of gravity.
Dropping the coin does not inherently prove gravity exists, the constant rate by which the coin accelerates is the indirect illustration of gravitational pull.
The above video is an incorrect oversimplification (as is always the case with these sorts of videos and people who love attention). Density and buoyancy do explain why lighter gasses rise above heavier gasses. However, all of the gasses are pulled upon by gravity as well (they too are accelerating toward the planet like anything else, it is their densities which come into play and determine their relative positioning). Gravity is why they don't float away from the planet off into space and why we are able to have an atmosphere like we do, unlike say... the moon or mars which are less massive and as such have a weaker gravitational pull.
The concept of density and buoyancy of gasses is also why very light gasses (see helium) will go up to the very outermost part of the atmosphere, can be energetically excited further and actually break free of the pull of gravity, resulting in their loss forever.
I am still not understanding your point. It's just not correct. Cheers.
Vsauce actually does a fantastic job explaining all of this: