The Generalized Processor Sharing (GPS) is an important service differentiation scheme in telecommunication systems and has attracted great research interests due to the desirable properties of fairness, traffic isolation, and work conservation. The major limitation of the existing analytical performance models of GPS systems subject to self-similar traffic is that these models have been restricted to a simplified scenario with only two traffic flows. To overcome such a limitation, this study proposes a new flow-decomposition approach to modelling GPS systems subject to multiple self-similar traffic flows. We present and prove the existence of a feasible ordering of GPS systems based on the required and guaranteed service rates of traffic flows. With the aid of the feasible ordering, we decompose the GPS system into a group of single-queue systems and then derive the queue length distributions and loss probabilities of individual self-similar traffic flows in the original GPS system. Extensive simulation experiments are conducted to validate the accuracy of the developed analytical model.