A liquid shader would make your characters swim through the gas.
I did this a few months ago. I had a cave connnected to a Water Pump from Oasis. I used a volume fog shader on a brush to create the gas atmosphere in the cave. The allies had to repair the pump to suck the gas out of the cave. I used a trigger hurt to cause damage. But since I didn’t want to use the ripping sound effect for the barbed wire, I used a second trigger multiple to trigger the sounds.
What I ended up with was a trigger hurt triggering every second, and a sound effect triggering every 5 seconds. I used a sound byte of Beavis screaming for the Axis, and Butthead for the allies. It worked really well.
It was rather complicated for me to set up and I eventually scrapped the idea for my map. For the most part, it worked fine, but the volume fog shader conflicted with the main fog shader for the entire map so sometimes it would not show up at all.