Exact closed-form performance metrics for mixed free-space optical/radio frequency (FSO/RF) communication systems are not available in the literature. This paper presents novel results for the performance analysis of a dual-hop FSO/RF transmission system where the FSO link is modeled by the Gamma-Gamma distribution with pointing error impairments and under heterodyne detection, and the RF link experiences the Nakagami-m fading. Using amplify-and-forward fixed gain relaying, new closed form expressions for the outage probability, the moments, the average bit-error-rate, and the ergodic capacity are obtained in terms of the extended generalized bivariate Meijer's G function. Monte Carlo simulations are provided to confirm the accuracy of the newly proposed results.