The ability to detect and recognise a variety of gases under certain environmental conditions has significant potential impact in many areas. Gas sensing has long received attention with microwave and infrared spectroscopy. With many molecular resonances occurring in the THz range leading to simple, unique spectral features, THz time domain spectroscopy (TDS) shows promise as a tool for gas detection and recognition. Our earlier work proposed a simple method that extracts the line positions from gas species without a reference pulse and classifies them by means of the minimum Euclidean distance classifier and the Submillimeter, Millimeter, and Microwave Spectral Line Catalog. This paper presents a preliminary investigation on a reference-free spectrum as an alternative classification feature for gas recognition.