We consider the physical layer security (PLS) of the generalized spatial modulation (GSM) enabled indoor multiple-input single-output (MISO) visible light communication (VLC) downlink, wherein an optical artificial noise (OAN)-assisted scheme is proposed to enhance the secrecy of the considered system. Due to the transmitter (Alice) sending confidential messages and OAN with the power and amplitude constraints, a truncated Gaussian distribution is exploited. With a finite discrete distribution of the input signals, the secrecy performance is analyzed in the OAN-assisted GSM VLC system. Specifically, the average mutual information (AMI) is first proposed, then the lower bound and approximation of AMI, and the achievable secrecy rate (ASR) are evaluated as well. In addition, considering the proposed power allocation problem of the considered OAN-assisted GSM VLC system, the particle swarm optimization (PSO) algorithm is explored to attain the global optimal power allocation coefficient, which is capable of maximizing the ASR. The numerical and simulation results validate our theoretical results of the OAN-assisted secrecy performance of the considered indoor GSM VLC system. (c) 2022 Elsevier Inc. All rights reserved.