Generalized spatial modulation (GSM) is a promising technique that can highly increase the spectral efficiency for ultra-high data rate systems. However, its performance degrades in highly correlated channels such as those in the millimeter wave (mmWave) and sub-Terahertz (sub-THz) bands. GSM conveys information by the index of the activated transmit antenna combination (TAC) and by the M-ary symbols. In conventional GSM, the legitimate TACs are randomly selected, where their number should be a power of 2. In this paper, a simplified EGSM (S-EGSM) based on TAC selection but without channel side information (CSI) is proposed for highly correlated channels. Moreover, an efficient Index-to-Bit mapping for spatial bits based on Gray coding is proposed to reduce the spatial bit-error rate (BER) instead of using the normal binary mapping as in conventional GSM. Simulation results show that the proposed TAC selection without CSI (S-EGSM) outperforms the existing method by 1.4 dB in highly correlated channels. In addition, the proposed S-EGSM when compared to TAC selection with CSI can be considered as a good trade-off between performance and complexity since it does not consider any feedback from the channel when updating the TAC selection, which are, in the proposed approach, determined offline. Finally, simulation results of gray coding for spatial bits show a performance gain of the order of 1 dB in highly correlated channels, and which becomes less significant in case of low spatially correlated channels.