For the Gas mask just use SparkIV to open ?:\Rockstar Games\Grand Theft Auto IV\Grand Theft Auto IV\pc\models\cdimages\pedprops.img and import the gasmask files. To reverse import the back up files. To get the camo clothes import the wtd's into playerped.rpf under the same directory with SparkIV. You may want to back up the originals if you want to reverse that. You'll need a helmet trainer such as native trainer to get Niko to keep the "helmet" on. Enjoy 

Native trainer http://www.gtagarage.com/mods/show.php?id=5646