Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has recently attracted much researchers’ attention due to its superior spectrum efficiency and reliability compared to the traditional OFDM. Cooperative decode-and-forward (DF) relaying has been incorporated with OFDM-IM, which provides a higher energy efficiency and better network coverage. However, it might not be feasible in realistic applications owing to the high system complexity and transmission delay rendered by complex decoding and channel estimation procedures. Therefore, in this paper, we propose a fixed-gain (FG) amplify-and-forward (AF) relay assisted OFDMIM system, which does not need to perform complex decoding and channel estimation at the relay, but only requires a preprocessing capability at the relay, e.g., cyclic prefix (CP) removal and re-insertion. Therefore, the system complexity can be reduced and the forwarding delay as well as power consumption caused by processing at the relay also decline. We analyze the average outage probability, block error rate (BLER), and achievable rate of the proposed system and verify all analysis by numerical results. The proposed FG AF relay assisted OFDM-IM provides a simple solution to the implementation of OFDM-IM in new network paradigms, where nodes are simple, power-limited and/or complexity-limited.