Surface acoustic wave (SAW) devices are ideal candidates for gas sensors due to their small size, low cost of production and high sensitivity. Increasing restrictions on pollution and emissions create the necessity for sensors that can operate in the harsh environments found in vehicle exhaust systems and industrial production. Gallium nitride (GaN) is a robust, chemically inert, piezoelectric semiconductor, making it an attractive material for SAW devices designed to detect and monitor gases in harsh environments. In this work, SAW devices designed to operate at the 5th and 7th harmonics are fabricated on GaN thin films and their performance is measured through insertion loss, signal to noise ratio, operating frequency and quality factor. Devices are directly exposed to the exhaust gas of a common diesel engine. Device performance is then re-measured and compared. SAW devices fabricated in this work have measured operating frequencies above 1 GHz, and quality factors up to and higher than 2000, depending on the harmonic mode. SAW devices on GaN showed good chemical stability and measured changes in device performance after exhaust exposure was negligible.