In this paper, we have designed and analyzed the performance of Gate-All-Around Cylindrical Nanowire FET (GAACNFET) with Indium-gallium-arsenide (In (1-x) Ga (x) As) as channel material and thin catalytic metal gate for the application of Ammonia (NH 3 ) gas sensing. Cobalt and Ruthenium are applied as a gate electrode due to their high sensitivity and reactivity towards Ammonia gas. In this device, when gas molecules are adsorbed over the thin catalytic metal gate then there is a visible variation in parameters like ON-State current sensitivity(I on ), OFF-State current sensitivity (I off ), ON-State current to OFF-State current Ratio (I on /I off ) and threshold voltage (V th ). Result shows that as the work function is varied from 0 meV to 50 meV, 100 meV, 150 meV and 200 meV for catalytic metal at the gate, then sensitivity also increases. Effects of different dielectric materials as gate oxide have also been studied.