Metro Exodus: How to Put on and Take off your Gas Mask ...
On PC it is the G button, and on PS4 and Xbox One it's down on the d-pad. You can't just press G or down though, you need to hold it to trigger the effect. Pressing it will just wipe away the dirt and grime. Once you hold it down though, you'll either put it on or take it off. If you aren't already wearing your gas mask, you will put it on.