This paper proposes an optimum mechanism to select appropriate gateway based on multiple node metrics: remaining energy, mobility, and number of hops, for interconnected mobile ad hoc network (MANET) with infrastructured network. To choose an optimum gateway node, we use the multiple criteria decision making (MCDM) method called simple additive weighting (SAW) to outrank the optimum gateway node. SAW method calculates the weights of gateway node by considering these three metrics. Node with the highest weight will be selected as a gateway. Simulation results show that our scheme can improve throughput performance, gateway lifetime, and packet delivery ratio of MANET and infrastructured network.