We derive and analyze a bit error rate (BER) expression of a Gray coded rectangular QAM (R-QAM) signal with maximal ratio combining (MRC) diversity reception over Nakagami-n (Rician) fading channels. The derived result is provided in terms of the Whittaker function and the confluent hypergeometric function. Because the derived expression is general, it can readily allow numerical evaluation for various cases of practical interest such as line-of-sight (LOS) or satellite communication channel analysis.