Since its acceptance as the adopted authenticated encryption algorithm, AES-GCM has been utilized in various security-constrained applications. This paper describes the benefits of adding key-synthesized property to AES-GCM using FPGAs. Presented architectures can be used for applications which require encryption and authentication with slow changing keys like Virtual Private Networks (VPNs). Three methods are selected to implement the SubBytes of AES to increase the flexibility of the presented work. Furthermore, we propose a protocol to protect the bitstream of the proposed architectures. Our architectures were evaluated using Virtex5 and Virtex4 FPGAs. It is shown that the performance of the presented AES-GCM architectures outperforms the previously reported ones.