Make a wall that allows all particles through, but blocks air currents. I don't know how many times I've needed that. Sure, it's unrealistic, but so is the wall that allows solids through but not liquids and gases, and if I cared about that I wouldn't use that.
I'm with ZebraineZ when he says that 'gas' is a bit generic, i think a good solution would be to change the current gas to MTHN (methane) or some other flammable gas and put in another element in from the noble gas group, like Neon, Argon or Xenon. The noble gas could ionize into plasma when electricity is put through it and would be conductive, it would not be flammable or reactive in any other way (just like the PT diamond of gas)
pressure isn't necessary, just proximity (which really only occurs under pressure). I recommend that its called Xenon (XNON), because the PT plasma is a purpley colour, the discharge colour of Xenon (also Xenon is just the most kick arse name and element)
silly pommy spell checker, thinks colour should be color.
Haha. I had to make the choice on the wiki wether or not to put colour or color. I decided because The Powder Toy is being developed by a British person.