Generalized spatial modulation (GSM) is a novel multiple-input multiple-output technique, which relies on a sparse use of radio-frequency front-ends at the transmitter. In this letter, a pair of detectors based on the layered message passing (LaMP) detector are proposed for large-scale GSM. Specifically, a threshold-aided LaMP detector (T-LaMP) is proposed with the purpose of reducing the computational complexity of LaMP detector. In addition, for the sake of achieving further GSM system performance improvements, a probability-ranking-aided LaMP detector (PR-LaMP) is also proposed. Simulation results show that T-LaMP detector is able to achieve similar performance as LaMP with lower complexity, while PR-LaMP detector is able to achieve considerable performance gain.