GS101-868M gas detector for gas heaters natural gas methane city gas Methane gas is an odorless, extremely combustible gas with the chemical formula CH4. GS101 (CH4) gas detector is also called natural gas detector. This kind of gas detector is usually used in boiler rooms, hotel kitchens or residential buildings for fire protection. Camping gas (propane) e.g. in caravans is not detected.