Ion beam irradiation dramatically enhances the sensing response of a graphene gas sensor by producing defects on the graphene layer. The number of defects depends on the ion size, energy, and dose. In addition, by using different gases such as ammonia (NH3), nitrogen dioxide (NO2), and carbon monoxide (CO), we were able to compare the sensing response of graphene with that of ion-irradiated graphene. The sensing response of ion irradiated graphene is 15 times larger than pristine graphene. For the other gases, the sensing responses are also enhanced. The sensing response ratios for the different gases are closely related to the theoretically expected adsorption energy between the graphene and the gas molecules.