While I was playing Trem with basilisk, I was thinking that it could be nice if humans can buy a gas mask from armory, to don't get infected by basilisk's gas. What do you think?

That would be pretty cool, though I thought that the battle suit did that?

Well, battlesuit can have his own protection. Gas mask can be taken by only light armour and helmets

I think that gas is little rare and new thing just for this would be underused. Some kind of mask can be already inside helmet.