The Gas Mask is exclusively spawned via the admin F1 console and is not available for player use in RUST. It can most notably be seen on both tunnel dweller NPCs and the newer Underwater Laboratory monument dwellers.
If you do spawn this item, it has identical defensive statistics as the Bucket Helmet and even shares a placeholder graphic with it.
While it cannot be crafted in RUST, it can be recycled for a minimal amount of Metal Fragments.