The gas filter replaces a part of your trachea and with the help of an electric chemical process, it filters harmful particles and substances from the air that you breathe, making you immune to many forms of gas, poisons, dust clouds etc. You can, for instance, breathe freely even inside tear gas clouds, sandstorms and other poisonous environments.
However, keep in mind that while the gas filter protects you from inhaling harmful substances, it does not provide you with additional oxygen. So if you are in a displacement gas cloud, you will still suffocate.