We investigate the performance of dual-hop cooperative amplify-and-forward relaying systems in wireless body area networks over independent and non-identically distributed Gamma fading channels. In particular, tight lower bounds on the end-to-end outage and symbol error probabilities, for fixed gain and channel state information (CSI)-assisted relaying scenarios, are derived in closed-form for arbitrary signal-to-noise ratios (SNRs). We also present simplified expressions in the high-SNR regime that enable us to quantify the system performance in terms of diversity order and coding gain. The numerical results provide important physical insights into the implications of model parameters on the system performance; for instance, it is demonstrated that the fixed-gain relaying has a better performance than CSI-assisted relaying, where the former has more realistic assumptions.