Much much longer barrel proportionally and the light gas (usually hydrogen) transfers momentum more effectively due to its higher speed of sound.

For one the reloading procedure would be a nightmare. That gas piston in the first stage probably gets extremely well lodged in that conical section before the projectile, and the whole barrel needs to be held at a vacuum, meaning you either need a very fragile rupture disc on the end or to convince your enemies to stand in the vacuum chamber the gun is attached to. For two, even though the velocities are relatively high, they're not remotely high enough to encroach on the territory of conventional munitions given such an incredibly light projectile. To give that 1 gram projectile the same energy as a modern long-rod penetrator, it'd have to be traveling at 158 km/s - 22 times faster than this gun currently does, or put differently 0.05% the speed of light. Lastly, consider that the initial charge for this light gas gun is a shotgun shell, meaning the maximal energy it could have is that of a shotgun shell. To give this thing the destructive power of a tanks main gun, you wouldn't miniaturize it, you'd macronize it, making it even larger and more unwieldy.

It has been tested as a weapon by the US by Utron but from what I understand there were feasibility issues with storing large amounts of gaseous hydrogen

You can find a link to my LinkedIn in the description, I would be happy to talk with you through there :)