A). Go to the Unity asset store and visit your My Assets. Download the asset if not already downloaded and click Import, import all required features of the asset for your use. It should have appeared in your project under Gas Mask System. (If Unity warns you that you may overwrite project data, do not choose the Project Settings tickbox when importing the asset)
Q). How can I manually setup this asset?
A). See the manual setup instructions on the Detailed Setup page