Nearly everyone is familiar with the images of the heroes and the villains dressed in gas masks that we see in the news. Basically everyone that we see in the news that is exposed to hazardous chemicals is wearing a gas mask. It does not take much to dress up as a soldier, S.W.A.T. member or terrorist. All that is required beyond the initial gas mask is a helmet and vest. Camouflage paint can make an ordinary gas mask, hunter's vest and helmet turn into a really neat soldier's uniform or terrorist's attire.
For all of the gamers out there, there's no gas mask costume that beats the Half-Life 2 combine soldier. Not everyone has played the game, and not everyone even knows what it is, but most people will recognize it to some degree. One of the coolest things about the game is the world building that the creators did. Part of the world they created was the faceless enemy soldiers behind their gas masks wielding guns and clad with body armor. You don't even have to be familiar with the game to understand the psychological effect these characters have on you. The costume is simple, consisting of something that resembles a bullet-proof vest, a fake gun, a helmet and a gas mask. Just paint or dye them all black and gray, and try to look tough. If you happen to have the option, you may want to tint the view ports of the mask. You can do it by replacing the view ports with sunglass lenses or by using window tinting.
If your gas mask happens to have a long breathing tube, it's pretty simple to turn it into an animal costume. A little bit of papier-mâché or foam and some paint can turn your gas mask into an aardvark, elephant or anteater. You'll want to remove the canister at the end, since you won't be needing the filtration properties, and paint accordingly.
For a mosquito costume, you will need foam or papier-mâché. Just apply the foam or papier-mâché to the breathing tube until it looks like an insect proboscis. This costume will be even more detailed if accompanied by a bag of red dye-colored water. Paint the view ports to look like an insect's eyes and you're golden.