I can't say that a gas mask isn't a totally bad idea. It depends which one you buy. Sweat is the big concern so I've only used my mask on cooler days.
The air goes in one way, and goes out the other, leaving little to no room for my breathing to fog up the eyes. (which by the way are extremely thick) But like anyone else would say, it's the sweat from the heat that'll fog you up more than your breath. My mask doesn't restrict my vision too much, but restriction is still noticeable. Aiming is a bit difficult with a bulky mask, but I personally save this for night games.
Overall, I'd say that a mask isn't a terrible idea if you're trying to play a certain role, or if you want a certain look. But I'd use some shooting glasses over my mask any day.