The immunity provided by certain gear is different from the critter power immunity. IMO, a gas mask means you don't have to roll the test against inhaled-vector toxins, provided the mask is well maintained, properly fitted, and in good working order. Now, if you were rushing to put on the mask (you see a gas grenade incoming, and pull it on, for instance), or were using a mask you just 'found', then I'd probably make you roll something to make sure you got everything on right