Gases coming in or not has been debated before and generally there's not a consensus.
I don't leave mine closed because of gasses, I leave mine closed because I want to flush my tanks. Just like the black, having insufficient water in the grey tanks will lead to sediment that builds up instead of being washed away. In the grey it'll be things like bits of food, hair, skin cells, etc... I want that stuff suspended in liquid so that it flushes away when I dump, instead of sitting on the bottom of my tank.
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There's no use crying over spilled milk, unless it's on your keyboard.