I've realized as of late that I'd really like to wear certain hats and still be able to deal with gas. My suggestion is to separate the two or put gmasks into a different category itself.
I think we should have two slots, one for each and also a layer manipulator so if a gmask looks better overlapping or not on a hat, it can be done. In a way this is like wearing two hats at once, but if you look past that you can see it may be necessary for the utility.
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Another idea I have is to incorporate an on/off button on screen if you have a gmask equipped.